You're missing the point of my question, however. Why is it that ignorance of the law is not a defense to a crime while ignorance of a VERY BASIC AND ALMOST UNIVERSALLY KNOWN Constitutional right is a defense to an allegedly "forced" confession?
So, if someone is ignorant of a little used law that applies in a certain fact situation, they can be convicted of a crime. If that same person is ignorant of their most basic rights in the face of government authority, we let them go free if they fail to exercise those rights and are not "reminded" of them.
Essentially, we're saying to people that you don't need to have any civic responsibility for your own well-being. As Ornery says, if there wasn't something to tell, the person would be fine. A criminal goes to jail for their ignorance -- not of the law, of their rights. Is it the fault of the police that the perp is stupid?