The wrinkle in this case is that the thumb print giver was in parole. The conditions of parole stated that failure to divulge phone pass codes on phones could result in arrest and phone seizure "pending further investigation". The parole conditions didn't say anything about forcible thumb print taking.
So the logic here seems to be:
If he had agreed to unlock the phone then the result would be the same.
If he refused to unlock the phone, that is a legitimate grounds for arrest. Fingerprinting is a routine part of being arrested, so there's really no harm if it's done on a phone in a patrol car. Either way, the result would end up about the same.