i had always thought it was illegal in college, just ignored.
The rule in college and the pros are a bit different, but looking into it they amount to the same thing. The gist of it is simple - you cannot directly push, pull or lift/carry the ball carrier in an effort to help him advance the ball. However, you can try to push/block a would be tackler off of the ball carrier. So it comes down to this - is another offensive player directly pushing or pulling on the ball carrier, or just the pile? Because if it is just the pile, you can't tell if there is a penalty or not because you could be trying to move a tackler, not the ball carrier. For that reason, these plays do not get flagged.
It seems that in the NFL there are fewer of these plays because of the size, speed and strength of the first defenders to reach the ball carrier. The play tends to end very quickly once the RB is wrapped up. Also, these guys are trying to play the game without getting hurt. A great way to get hurt is to have someone hit you from behind in a scrum.