This isn't something you ought to have to learn in a free and just society, just as you shouldn't have to know the going rate for a police officer in your town.
There is a lot of talk about the disparity of justice in America and how we are not actually so free after all. I believe most of this feels good to say because we know its true, but after we leave the seminar we go back to a standard civilian mindset.
What I mean is that we walk and talk out in the world like America is just and the police will be fair with you; it is only when we sit down and examine the system that we realize intellectually that this is not the case. But this epiphany fails to carry down to the reptile brain once we stand up again.
Changing that "walking around" mindset is the first step of self-defense training. It is necessary when dealing with American law-enforcement.
Although I agree that it ought not to be.
Law enforcement often use tactics like this to convince citizens to voluntarily testify against themselves. It's not fair as the tactic preys on our natural desire to help. But it's important to train ourselves into the mindset that if you are talking with uniformed law enforcement, then you are being interviewed by them.
To whit, if you are being interviewed by law enforcement, your best option is to never answer any of their questions and repeatedly ask to leave. If you are not able to leave then you switch to repeatedly asking for your lawyer.