The issue is the 5th amendment right to not incriminate one's self. No warrant can force you to incriminate yourself.
Your fingerprints are fairly "in plain view" anyway, so quite accessible. Having been arrested, the defendant would already have been compelled to give up fingerprints. Sure, your prints may unlock a phone which could be incriminating, and is thus a 5th Amendment concern, however, if you kept photos in a locked chest and kept the key on a necklace, would it be against your rights to use it to unlock the chest when conducting a lawful (warranted) search? If you think that would be in violation, what about using a photograph of that plainly-visible key to create a copy and thus unlock the chest? Divulging a password could border on self-incrimination (giving self-witness that you truly had access to the passworded resource in question), and can thus be more clearly protected by the 5th amendment.