Officials on the field need to officiate, and should never count on replay to clean things up. The official's job is still the same as it always was, simply put "Call them as you see them."
Replay is not 100%, and by putting the flag down, you are saying that in your judgment that was a foul. It is not appropriate to call a foul if you have no doubt that the foul did not occur. It is wholly inappropriate for an official to call a foul that his judgement says is not there.
This is about integrity.