What if the code found in github with MIT license was stolen by someone and uploaded there? What would happen if a piece of code from a proprietary source was uploaded by someone without permission to github with the MIT license and thousands of people have used it. Even if the company who owns the code requests github to drop the repository, what would prevent the endless spread of the code by users believing it to have MIT license. What should the owner do in such cases to prevent it from happening?
If it was something important to the owner, they would file injunctions/suits against everybody that they find the code in, and make those very public. This isn't unlike what the RIAA did to individual downloaders of pirated music files, filing over 18,000 lawsuits. They could subpoena GitHub for information related to IP addresses for who downloaded the code and go after them. It is unlikely that they would get a financial award but they could have the court order them to stop using it. Eventually the RIAA found that filing these lawsuits was more trouble than it was worth and started going after the internet service providers. These reduced the number of lawsuits and forced ISP's to moderate its own users, blocking some sites and protocols that programs like Napster relied on. Basically they shifted responsibility from themselves to the ISP's.
To use an API over a network connection (as opposed to, e.g., the Windows API), a user communicates a request to the API host, or server, and awaits a response. The host of the remote API can refuse to serve requests from users for probably any reason. Such services often require users to accept a license as a condition of using the service, and they may charge a fee as a condition of the license. A license to use a service is obviously not necessarily bound to a license for the use of its source code, just as the ability to reach the service is not dependent on the ability to see (let alone use) the code in any format, whether it be the source code or some compiled form of the code. In essence, source licenses and service licenses have different primary goals, at least inasmuch as the source license seeks to restrict someone who has actual physical access to compiled code, and possibly source code. Service licenses do not have that concern, though I have seen service licenses that also prohibit decompiling. This is probably the result of a CYA attitude among lawyers: the language is already in the standard software license text, and it doesn't hurt anything to leave it in, and it could help if a service user somehow managed to download the program code.
Yes, assuming the material was given the standard license. You would be creating a derivative work, and only the copyright owner has the right to authorize creation of a derivative work. See the copyright FAQ for general information. The owner can file a DMCA takedown notice with YouTube and they will by policy notify you of the infringement claim and unless you file a counter-notice (you legally couldn't given the facts you're asserting), they will take it down. The owner can also sue you.
Unless you have a legally valid IP right related to the specification that statement is meaningless. When a software license is granted it is based on the copyright of the code. The copyright of the spec. just stops people from copying the spec - it does not protect the information in it. You can restrict copying of the spec. under copyright, you can make up a name for the spec (like USB or Bluetooth) and get a trademark and only allow the trademark use in limited cases(doesn’t stop implementation of the spec), or get a patent that would be necessarily infringed if something complying with the spec was created and used, sold, made, etc. or you can keep it secret and only show it to people who contractually agreed with your terms.
No, it means you can't copy it. By default, the copyright to a work is owned by its creator, and nobody else is allowed to copy it, or create derived works, without their permission. That permission can be granted by a license. "License unknown" doesn't really tell us anything, but it certainly isn't clearly granting you permission. So you don't have permission to copy, and thus you cannot. You would have to seek permission from the copyright holder. See also If no licence is distributed with an application/source code, what license applies by default if any? (Some jurisdictions do allow for "fair use" exceptions, which allow you to copy a work without permission. You haven't said what jurisdiction you are in.)
It does mean that you cannot reuse any parts of the source code, even small simple ones. You would have to rebuild the code from scratch. There is a significant chance that the code would be "substantially similar" to the code that you were hired to build, also that if anybody else were to write a bubble sort or 24-to-am/pm conversion routine, it would look the same, where even variable names (which should describe function) are the same or very similar. In case of an infringement lawsuit, you would have to defend yourself by showing that there are only a few ways to code a given function. Copyright protects only the "expression", not the abstract idea. A linked list is an idea, which can't be protected by copyright; same with recursion, pointers, stacks, object-oriented programming... Anything that involves copy and paste is infringement. If you re-read the copyrighted code and then try to reconstruct it, you probably crossed the infringement line. If you remember the problems and solutions and accidentally write somewhat similar code, that is probably on the safe side. From the perspective of the programmer not wanting to always reinvent the wheel, it would be most useful to make a distinction in the contract between "the essentials of the customer's program" versus "incidental utility work". The difficulty will arise in saying specifically what is essential vs. incidental. For instance, I know that if I were to hire you to develop a speech-recognition system, low-level audio-acquisition and encoding would not be essential to my purpose, whereas DSP parsing routines would be the center of my interest. The programmer would then want to retain recycling rights to all non-essential code.
There seems to be no controversy about whether you are allowed to make a copy of a publicly accessible GitHub repository by forking it. As they agreed by accepting the GitHub terms of service and making their repository public, the repository owner grants that and a few other rights to all GitHub users. The actual question seems to be whether you may then publish the URL of your fork. Of course you may, to the same extent that you may publish anything else. The URL is data, not a creative work, so copyright does not apply to it. The considerations are the same as for publishing URL of the original repo. There are things you might say about the URL or the repository it identifies that could get you into trouble, such as claiming rights you do not have, or misrepresenting the rights that others have or that you can grant to them. Or publishing false and derogatory remarks about the code might conceivably land you in hot water. But the URL itself is not an issue.
Using it without permission is copyright infringement and illegal. Legally, you can try offering money to the company for the copyright or for a suitable license. For example offer them $1000 for a copy of the code licensed under the GPL license. If they accept, you are fine.