Cant really say it bothers me too much. Game was over as a contest. People are welcome to leave early. You dont get brownie points for sitting there until the end. Personally speaking, I make a point to stay until the whistle, just out of personal habit. Not because it makes me feel superior to people.
You really want to hear something absurd? I was at the US Open Cup Final in Kansas City last summer when Sporting Kansas City played the Seattle Sounders. Excellent game, great advert for football in the US. It went to penalties, and some people were leaving BEFORE THE SHOOTOUT. That was a major WTF moment for me. Why would you do that?