Andy,
I was the one who called this specific play dirty so let me explain.
I have no problem with someone lowering their shoulder the way the cover man did if the player is not defenseless. Had he been the ball carrier or even a WR going over the middle for a catch (there was no helmet to helmet so this would be fine),
Where the problem occurs is that this was a defenseless punt returner looking up for the ball in which he is protected by both a fair catch called and a halo of space in which he is granted to catch the ball.
I also understand that if you are being blocked you want always be able to control yourself from not making contact but this is not the dirty part. The dirty part is that the player either new he was being blocked and thought he could get away with it or just ignored the halo and fair catch rules either way to me it is extremely dirty.
Had he just bumped into the player because he was being blocked nothing wrong but to lower shoulder on someone who is not only protected by the rules but also who is not even looking is dirty.