It is illegal, it just hardly gets called. I have only seen one call against the offensive player called and it was a game this year, don't remember the teams, however.
What I get tired of seeing is a hit like we saw on Dowler in the playoff game where the defensive player is coming in low and the offensive player lowers further, into the hit, and it draws a flag. Further, he then acted like he was knocked out and that, IMO, further swayed the decision to call targeting on Harper. Then he goes to the sideline and I don't think he even went in to be looked at, b/c he was back in the game two plays later and we were just supposed to accept that the hit was dirty and instead of a fourth down stop we had a big momentum shift that probably decided the outcome of that game and a national championship.
Too big a call there to just say that is targeting on a quick review, but the offensive players are being coached to do all of this.