I'm a Chiefs fan. I don't bet against them in a game that counts. I don't want to sit there rooting against them. I'll bet against them in the exhibition season, or in a meaningless game at the end of the season where they rest their starters because it doesn't count.
I also think when you are emotionally invested in a team, you don't see it as clearly as when there is no emotional involvement. Doesn't stop me from using their opponent in contests, which I've done lots, but it's not the same thing.