Actually, based on a 2014 case out of Virginia, they can. However, police cannot make you enter a code into your phone.
http://blogs.wsj.com/digits/2014/10...be-required-to-unlock-phone-with-fingerprint/
I do not agree with this judge's ruling but my opinion doesn't count for much, he is the one with the black robe. Police have always had the ability to "search" a suspect they detain for safety reasons (i.e. to make sure the person they are detaining or putting in the back seat of their vehicle isn't carrying a concealed weapon). Police routinely make suspects empty their pockets and they often go through a person's wallet to get their ID. Some officers decided on their own that their ability to "search" a suspect extended to searching a suspect's phone (call logs, voice mail, text messages, photos, etc.) and this has been challenged in court with seemingly contradictory rulings coming out of different courts.
The court cited in the WSJ article above decided that since police have the ability to collect a DNA sample (cheek swab) of a suspect, it makes sense that officers should have the ability to compel suspects to put their finger on their phone to unlock it (again, I adamantly disagree but that is irrelevant).
The practice of requiring suspects to unlock their phones so they could be searched was also challenged in another court. I don't recall the exact details but in that case the suspect had a pass code rather than finger print scanner and the judge ruled that the demand to unlock the phone was an "unreasonable search and seizure" because the purpose of the search is supposed to be for officer safety (i.e. look for hidden weapons) and that whatever may be on a phone is not a hidden weapon.