I think this actually applies to a lot of popular media. One of the strange twists of modern popular culture is that we have come to consider "indie" to be cool, but really expanded that idea, so that the things our society considers cool are the things that are considered to have been unpopular.
Halo, being somewhat of a cornerstone for the modern shooter genre, and one of the most popular games of all time, then is an iconic symbol of what is not indie. That is to say, people think it is cool to say that they hate Halo, because it becomes a face for the old cool that is considered corporate or manufactured.
Personally speaking, I find it rediculous. I can't imagine any reason to hate Halo, even if I didn't enjoy it (which I don't, it's one of my favorite games to play online with friends), I wouldn't feel the need to hate it, because its fanbase does speak for its merit, and because it's done nothing to offend me.
