I'm not saying there isn't a line to be drawn - obviously given your example most people would shrink away from the idea of using someones thoughts to convict (literal thought-crime).

It's a question of where we draw the line. If we don't allow a judge to issue a search warrant for a phone where there is reasonable suspicion of illegal activity - why should we allow a judge to issue a warrant to search a car/property/paper-work?

Of course that completely oversteps the line in the other direction though. Conviction rates for many crimes would drop through the floor if the legal authorities have no way of obtaining evidence.

Given how powerful smart phones have become and you come run a criminal enterprise solely through your phone, it is reasonable to expect that the police with a proper warrant would be able to inspect your phone.