I’ve decided it’s a tribal thing. Take my family for instance. They root for the Red Sox and the vote RAT. Neither tendency is based on logic. You root for the Sox cause you live in Boston, and you’ve always rooted for the Sox and you hate the Yankees. You no more would pull the lever for a Republican than you would go out wearing a Yankees cap. This is the best I’ve been able to do. Other than this there is really no explaining it.
You're right. It is a tribal thing. People don't think things through first. They get on one team or another first, and make up the reasons why afterward.
Often enough it's not what one's own team promises, but what one fears the other team will do that determines where one ends up. There's more focus on what someone thinks the other side is going to do to make things worse than on what one's own team will do. The team someone chooses is just assumed to be good because it's against the "bad guys".