It was a deeply divisive event - for a very large fraction of the playerbase, it was the most fun they've had playing WoW. The entire way the game functioned changed for a week - some people set out to grief, others worked to try to cure people infected. And it was done just before the release of a new expansion - any gear or progress that occurred in that week would be invalidated two weeks later when the new expansion raised the level cap by 10 - all of the best equipment you could earn prior to the new expansion would be replaced by normal leveling gear. The game was, so to speak, in a lame duck state at the time, so why not have some fun with it?