Most do it now to look cool. It use to be fairly common on racebikes back in the 80s and 90s and I believe it was done to soak up any leakage but I'm not positive on that reason but I do know there was a practical reason back then. Now it's just a poser thing.
Just because a bike is pink doesn't mean the rider or bike is gay! My bike is case and point lol I've seen the sweat bands once or twice as well, but on harleys and import cruisers. I thought it was to catch leakage as well. I haven't seen it on any SB's though, I'll have to keep an eye out.