Actually, in stadium sports, teams are expressed in plurals. So " I'm a Laker fan." is technically incorrect if you are an NBA or NFL fan. In fact, that holds true for "Chicago Bulls", "Dallas Mavericks", and " Miami Dolphins" etc...
Yes. Even in the Bay Area, I even heard the locals say " I am a Niner fan ". Sports fans can be someone who is a lawyers to a street thug, so their expressions and language can also change drastically. It was just my observation. That's all.