The Definition of "Unnecessary Roughness"

I've been saying this all year:

The manner in which Gates was injured raises a broader point regarding player safety. It was noticed over the weekend that plenty of ball carriers were being hit while down, most often by defensive backs. (The problem was most prevalent in the Jags-Steelers game, coming from the Steelers players.)

If one of the league's goals is to protect players, why aren't flags flying when bodies are flying after players are on the ground?

I'm not sure when the problem started, but it's much worse this year than it has been in a long time.  And it's not just dirty players like Rodney Harrison -- although he's still doing it as much as he always has.  It seems like at least once every game a guy from the secondary will come up and tag a player who's already down and making no attempt to move forward. 

I have no idea why this isn't being called, when it's clearly described as "unnecessary roughness" in my copy of the league rulebook:

(d) running or diving into, or throwing the body against or on a ball carrier who falls or slips to the ground untouched and makes no attempt to advance, before or after the ball is dead.

I've been saying to friends for awhile that it was obviously going to take a high-profile injury before the league would do anything about it.  We may have that injury -- even if, ironically enough, the Gates play wasn't really the best example of this foul since he did indeed appear to make an effort to scramble forward on the play.