I think some people might be commenting they aren't a fan because people might expect them to make mention of his death and need to explain why they aren't speaking up. I'm a huge music fan, grew up in the 80s, and I'm sure I listened to Michael Jackson at some point. But I don't consider myself a fan and so his death hasn't quite struck me the way it's struck others*. I don't feel a need to mention it on my LJ or participate in the forums that have posts about him (well, until now)...but if someone asked me why I wasn't saying anything or participating, my honest answer would be that I'm not a fan.
I think I know maybe 5 songs off the top of my head, and one of them is Bille Jean (which was covered by other bands), one of them is Heal the World (which was my 8th grade promotion song), and the other three are Beat It, Thriller, and Black & White. That's about it. Hardly makes me a fan in my eyes.
*But I do think it's sad that he died. I respect him as a musician, regardless of his personal life. Like any celebrity, I don't really think his personal life was any of our business while he was alive, and it's not now that he's dead.