It's not really much different than people who do the same for anime or video games they don't like, which is half the reason I don't visit the former's forum.
Nor is it as if people don't have a reason for feeling this way. Of course, there will be your trolls and you bandwagoners, but there will always, ALWAYS be more sports fans than there are sports haters. Always. No exceptions. The internet is but a vocal minority, and given that 9 times out of 10 you're going to meet someone who liked sports anywhere in the world, the internet's pretty much the only place people can vent. Because it's so ingrained in general culture worldwide that it's practically inescapable. Especially growing up, because in the same light, you're 99% more likely to be born into a family that's into sports than not. And if they don't force you to play, the education system will.
So it's not like it doesn't make sense. People like to be heard by people who have similar mindsets, and they also like to vent at the people who enjoy them on the internet because that's the only time when people can say such things without being berated. And I know this because I was a more vocal hater of sports when I was younger because I couldn't do anything about it. Today, I'm indifferent to most sports and repress my annoyance with overpriced tickets/overpayed athletes and hatred for various sports (Football and Baseball, for various reasons), among other things. Because I believe that people can like what they like, just that they shouldn't force others to feel the same way. Plus I now have a pretty broad view of what a sport is.
...Though it does bother me when people get elitist over what qualifies as a sport and what doesn't. That I will be vocal about. But do you know what REALLY pisses me off? That fact that I had something to say and that my internet decided to go on for an hour, so by the time I post this we'll probably be in the hundreds, I'll have to edit the damn number, and I'll have missed the entire ****ing conversation, which pisses me off because this is something I actually like to talk about, especially with people on both sides of the fence (and on the fence) which, for me, is a very rare occurrence. And I won't edit this because I'm sick of this damn internet going out every five seconds.