As a coach, I have a slightly different take.
1. If they are doing it intentionally to try to hide a wideout, flag it.
2. Technically, it's not the action of the defense that makes or not makes it a foul. But if the wideout gets covered by the defense (without having to be disadvantaged by rushing into position), no need for a flag, but after the play, "remind" the coach the player needs to be inside the numbers after the RFP. Here's why: later in the game, that player is going to do the same thing, and the defense won't see him, and now you have to flag it. If you didn't say something earlier, you aren't going to look very good flagging this one, when you did nothing before.
So even if the first one doesn't deserve a flag, "protect" your right to call it later with a friendly reminder to get it right.