Through the years, society has slowly downgraded the word fan (or fanatic). Once upon a time, it meant devoting one's fanship to one team in a particular sport or one athlete within a particular sport. It wasn't unlike honoring an unwritten tradition.
As time has passed, more people began the practice of disregarding that tradition. The general perception grew that since a team or athlete didn't owe fans any special treatment, why should fans think any differently?
For some, fan loyalty has morphed into a form of convenience not unlike knowing you have 'someone on the side'. Sure, he says that he "loves" his wife, but the first chance he gets, he's cozying up with his mistress because he "likes" her too. It's the best of both worlds, but it only proves the lack of devotion one claims is most important to him or her.
So, nowadays, it's, "I'm a fan of Team A and Team B!" or "I've been a fan of Team A for an ungodly long time, BUT I'm a fan of Team B now, for shizzle!" or worse (which is even more sad in my opinion), "I'm a fan of Team A and Team B and Team C and Team..!!!".
It really boils down to one's personal choice, but why people claim that they are fans when they aren't fanatical for a team is beyond me. Certainly, you can "like" as many teams as you possibly want. You can watch this team for x-amount of time THEN jump to another team x-amount of time and visa versa.
There's absolutely nothing wrong with that at all, but by definition, it doesn't make someone a fan, but it does identify someone as being a nomad. If you want flexibility, convenience and freedom to jump from team to team, you're enjoying your favorite teams of the moment. In other words, you wander back-and-forth and can't settle on just one team. On the other hand, if you're devoted to one team, you're fanatical for that one team. In short, you're a fan.
There is a fundamental difference in personal devotion. Why do people, who aren't really fans, dislike others pointing that out?