I don't like to play this card, and it might get me a vacation, but I've spent too many years and far too much damn money on that campus for anyone to suggest I'm not "actually" a fan because I leave early. If this were an in person chat, you can be assured I would not take kindly to such an assertion. If I spend my hard-earned money on a ticket to watch my alma mater play ball and for some reason I need (or choose) to leave a little early in a game where our own coach is hoping our players don't score, I'll have words with any man who suggests I'm not actually a fan. How dare you.
And yes, I've left sets by some of my favorite bands, too. Life happens.
Why not write a check, then sit home and watch the game. That way, you can still support the team, and then someone else can sit in your seat who actually wants to sit through the game. Heck, I'll consider you a good fan if you support the team without doing anything to their detriment.
This goes for everyone. You can all write a check, and support the team financially, and be secure in your fanhood. But, you go to the game, and you have a poor showing as a fan? Then you deserve to be called out for it. Obviously, sitting through four quarters isn't for everyone, so if it's not for you, but you're good with the money part, do that, I applaud that, but you leave an empty seat while players are still playing? That's rude.
Clearly though, for many people, and I've seen it expressed here many times, this is about entertainment, and isn't about supporting the team. They want to be entertained, and they get up and leave when it's not entertaining. Those are not fans, anymore than I'm a fan of Tom Cruise. I watch his movies if I find them entertaining, but I don't even like the guy otherwise.
Edit: My dad was a pastor to, and he wouldn't go to games because he thought the music they played was sinful. I respect people for their beliefs, and I'm not trying to start fights here or anything. I'm just saying, going to the game isn't for everyone, and there's a waiting list of people that would very much like to go and cheer for the team.