In today's game, the difference in a receiver making a play vs. a DB, or not being able to make a play against a DB, is sometimes a mere "half of a step."
If, for example, the nose of the football is touching a yard line, and the wide receiver's front foot is touching that same yard line, he, IMO, is in the neutral zone and has gained that proverbial "half-step" advantage even before the snap takes place. While I agree with the make-him-legal philosophy, he'd better expect that once he is told to back up, we're not going to repeat the message down after down. Gaining an unfair advantage is not.... fair. Following that initial reminder/warning, all it takes is one flag and he's legal for the rest of the night because he knows he is being watched. Been there, done that? You betcha!