It's true that even the sophisticated bad guys will accidentally leak info and leave a trail that can help police, but it's also true that mass surveillance significantly increases chances for false positives. The idea of all law systems in the world is to first protect innocents, and only then punish the guilty. Mass surveillance works vice versa, they know that it will affect a number of innocents by mistake, but are ready to accept that in hope that it might make police work easier.
