The penalty is what it is, regardless of the result of the play. Recall the play in CLE vs. KC a few years back... the player "thought" the game was over because he saw 0:00 (and ostensibly would not have taken off his helmet if he knew the play was still live), but the play had not ended. That does not excuse the penalty.
If what you are claiming were true, Harrison could have turned around and lit into Warner after the play had ended. I mean, since he thought the game was over and the penalty wouldn't make a difference (PIT would still keep the ball and kneel it out), it's OK, right?
Personal fouls and unsportsmanlike conduct penalties don't work like that.
Exactly. I recall the Dolphins-Niners game earlier this year. With just seconds left to play, and the Niners facing fourth down, Joey Porter busts around the end and sacks Shaun Hill. He then proceeds to rip his helmet off and run to the sidelines in jubilation. He was assessed an unsportsmanlike conduct penalty, but the Dolphins kept the ball because it happened after the play was over.