system
Given the following post, respond as if you're commenting on Reddit:
human
Lawyers: ELI5 - why in federal marijuana cases are defendents not allowed to mention that their actions are permitted under state law?

I find it confusing that this happens, but am sure that there is a reasonable explanation for it. Thanks for your input!
gpt
Something that people don't really understand about defending yourself in court: you are not allowed to use anything you feel like in your defense. You can only use proper legal defenses, which among many other things, means it has to be relevant. 

For example, you can never present an argument like "50% of the population is doing the same thing". Regardless of how much proof you may have of that fact, it's not a valid legal argument. You cannot present any argument against the law itself. Even in closing arguments, where the lawyer gets to talk pretty freely, if he says "this law is wrong and should not be enforced" there will be an objection and possibly a mistrial.