There's a knock on effect that is completely out of your control though, ok the person behind you appears as though they just don't want to stand but could if you stood in front of them, you can see that, they aren't visually injured/disabled/elderly/a child but what about the person behind them or the person behind them etc. can you guarantee that if you stood up every person who is forced to stand as a result is standing in front of someone who can just as easily do 90 minutes on their feet?
It's not as easy as you'd think, you'd be surprised the number of young people in block 3 who are forced take a breather each game let alone somebody who isn't as young and healthy.
It's a courtesy to sit down and await for the person behind to stand, if they do fair enough you're pretty much ok but they sit down you have no idea of the reasoning or who is behind them and for a game like this where people have paid a fortune for a game they've been waiting 20 years for, nobody honestly wants to be the person who missed it because a chain of people stood in front of them and they had no way of preventing it or joining them.