Your favorite MMO is a bit more of a commitment than choosing to eat an apple or banana. An MMO is a heavy time and cash investment, and as a result people get more attached to their favorite and more worked up when they feel someone is slighting it. The reverse is true as well: if you choose not to invest your time and money in a game, you can just...not invest your time and money in it and move on. The end. We all know, however, that a huge percentage of gamers are unwilling or unable to do so. A comment from We Fly Spitfires put it very simply: "People love to belong to one camp or another and irrationally abuse anyone who doesn't agree with them."
It's that sort of tribal thinking -- you're either with us or against us! -- that Tobold explores in his blog entry. It's an interesting read, and worth the time to take a look at.