Can a U.S. state legally prosecute immigration offenses within itself? Suppose a state passes a law that says persons who are not authorized to be here under United States immigration or citizenship law must leave the state or face arrest and prosecution by state officials unless US immigration authorities take them into custody. In other words, if immigration authorities won't arrest and prosecute/deport illegal immigrants in a state, the state will take its own actions. Would this be legal?
No. Arizona tried passing a law that, among other things: criminalized failure to comply with federal alien registration requirements, criminalized working without being authorized to work in the United States, and authorized state officers to arrest aliens without a warrant if they had probable cause that the alien had committed a crime that made them deportable. All three provisions were struck down in Arizona v. United States. The federal government has &quot;occupied the field&quot; on most immigration issues. That means they've regulated it so extensively that there is zero room for states to act independently. One of Arizona's laws that was struck down exactly duplicated a federal criminal statute, but even that went too far by allowing the state to apply its own enforcement priorities and prosecute cases the federal government would not. If a state made it a crime to be unlawfully present (which is not a federal crime), that intrudes even further on the federal immigration scheme. This doesn't mean a state can't alert the federal government to people who are unlawfully present. It doesn't necessarily mean state officers can't arrest for federal immigration crimes: a previous Ninth Circuit decision held that Arizona officers could arrest for federal immigration crimes on the same basis that they could arrest for state crimes, and the Supreme Court in Arizona v. US explicitly didn't address the question. However, if state officers make an arrest for a federal crime, the federal government still gets to decide whether or not to prosecute. What you're asking about would remove that federal control, so it is preempted by federal law.
They are not given independence from statute. This clause just says that conviction is not the end goal of the prosecutor. If in light of the evidence, the prosecutor comes to believe a person is not guilty, they are not to proceed with the prosecution. They must not hide exculpatory or mitigating evidence in order to get a conviction.
You have a couple major misconceptions about US law. First, crimes against the person are generally punished at the state level. States are not restricted to any sort of enumerated powers, and can pass any law they want to promote the general welfare unless there's a reason they can't. This is called the "general police power," and it lets them make everything from contract law to laws against murder. The federal government has to justify what gives it the authority to pass a law, and cities and counties have to justify their authority with state law or a state constitution, but a state government never has to preemptively justify why they have the authority to pass a law. States are especially not limited to powers listed in the federal constitution. The US Constitution sets up the federal government. State governments are set up by state constitutions, and derive their authority directly from the consent of the people of the state exercising their right to democratic self-determination. The only powers the US Constitution gives to states are minor technical powers involving state-federal relations (e.g. deciding how their presidential electors are appointed). But as I said, they aren't generally limited to any sort of enumerated powers by their state constitution either. Even the federal government isn't limited to "protecting rights listed in amendments." That's very little of what it does, in fact. Congress has powers listed (for the most part) in Article I and Article IV. It can pass laws banning murder in DC because Article I lets it exercise exclusive jurisdiction (meaning general police power) over DC and over federal enclaves. Article IV lets it exercise general police power over US territories, and pass laws regarding other federal property (I think it has a general police power there too, at least according to current law). The Necessary and Proper clause gives Congress the power to protect its own operations by, for instance, criminalizing the murder of a federal judge. Etc. Where there isn't a clear thing that lets the feds regulate something, they can probably get away with cramming "in or affecting interstate or foreign commerce" in the law, secure in the knowledge that practically everything affects interstate commerce. I'm not sure where you got the idea that laws are passed exclusively to enforce rights protected by the Constitution. They are not. They are not passed primarily for that purpose. Such laws do exist (e.g. deprivation of rights under color of law, which was passed pursuant to the 14th Amendment), but they're protecting you from government infringement of that right.
As user6726 notes in an answer, the page you link to derives from 26 USC 7701. However, it does not reproduce the text accurately. There, "United States person" is defined at section 7701(a)(30), and it notably lacks anything corresponding to "any other person that is not a foreign person." It's possible that that language is motivated by some court decision, but it's also possible that someone just added it for the sake of symmetry with the definition of "foreign person" without thinking about the logical paradox that it might create. Looking at section 7701, I don't see any explicit mention of US non-citizen nationals. It appears that such a person who does not live in one of the 50 states or the District of Columbia falls under the definition of nonresident alien at 7701(b)(1)(B) even though such a person is explicitly not an alien under the Immigration and Nationality Act. I do not see any regulations correcting this oversight, but I suppose that in practice such people are indeed treated as US citizens. I don't know enough about the classification of nonhuman legal persons as foreign or domestic to have any ideas about whether there are similar ambiguities there.
Absent newly enacted law in response to the end of the separate sovereigns doctrine, the first case to which jeopardy attaches (generally speaking when a jury is sworn and the first item of evidence is presented to it) would bar subsequent prosecutions, without regard to whether it arose in state or federal court. It would be a race to the court house rule. This is how it works if there are multiple prosecutions within a state, for example, one commenced in a municipal court that is not part of a state court system, and another commenced in a state court of general jurisdiction.
I have encountered this problem in Pennsylvania. The PA Code requires a District Attorney to approve all private criminal complaints. If the DA declines to prosecute, then an affiant can petition the Court of Common Pleas to review the decision. However the affiant bears the burden of convincing the court that the DA abused his descretion in declining to prosecute, which is a pretty high hurdle. In the United States the only other legal appeal I am aware of is through federal courts under broad federal laws like 18 USC 242 or 42 USC 1983.
Whether or not one holds the opinion that the adult morally should be prosecuted, there are only two legal questions – can the adult be prosecuted, and must the adult be prosecuted? The easiest question to answer is the &quot;must&quot; one – prosecution by the government is always discretionary. It is settled law that the government can decline to prosecute a crime. The decision to prosecute is entirely political. It seems likely that the government can prosecute, if they conclude that the action was not legally justified under §418 of the Crimes Act 1900, as defense of another against a criminal assault. The government won't prosecute if they conclude that such a defense is likely to succeed.
In the United States, there is no potential liability for the municipality or the police department. There is no legally enforceable duty of police to act to prevent either violations of the law, or apprehend criminals, or to prevent suicide of people who are not in police custody. Other countries have different laws on this subject.