The fingerprint is not communicative. That's the crux of the matter. The state of law, currently, is that if the police have you, and they have your phone, and they have probable cause that your phone contains evidence of a crime, they can compel you to press fingers to the phone.
It's no different, really, then that they can force you to press your fingers onto a sheet of paper for booking during an arrest, or to take a buccal swab.
What they apparently cannot do is force you to admit what if any finger of your would unlock the phone. Apparently some people use tricks like locking the phone with a knuckle or even the tip of their nose (that one sounds comical). Really though, it goes to show that the good old fashioned memorized password has not gone out of style. Any biometric or dongle-based authentication or encryption can be circumvented by legal process.
And in a civil matter, judges and parties have even more power. Ultimately, if a court is convinced that you know a password and are not properly invoking a 5th amendment privilege against criminal self-incrimination, you could be held in contempt (fined daily, or jailed) until you provided the password or the password-protected materials. At this point it could well become a battle of wills and proofs, which the contemnor may well lose.