Didn't Get the Memo?

I'm watching the Magic vs. Celtics right now and there's a person in one of the front rows wearing a Dirk Nowitzki jersey. I see this sort of thing a lot in all sports. There's always someone in the crowd wearing a completely irrelevant and unrelated shirt. I always wonder what people are thinking when they get dressed to go to the game. Dirk has no connection to either team... in fact, it's even the wrong conference. If I go with friends to watch a Knicks game and they're playing someone other than the Bulls, I'm not going to show up wearing a Ben Gordon shirt! If I'm not for either team, I'll wear plain clothes. Jerseys other than the 2 teams playing that are OK to wear are... former jersey of one of the players, college jersey of one of the players or Olympic jersey. Everything else just doesn't make sense!