Rights of the accused seem to function mostly as insurance against
law-enforcement error. If the entire law-enforcement process as a whole is
100% accurate, then there may be no reason for the accused to have
rights. But 100% accuracy is impossible. Even if the probability of error
is very small, it may be that most people are risk-averse enough to want
the accused to have the right of refusing being scanned.
Another possible answer is that we want law-enforcement to be less
efficient, not more. In an ideal world where all relationships are
consensual, no law (unless it's unanimous) would be enforceable.
If you could extract a single bit from someone's mind-state, what would
the bit encode, and whose mind would it be?