You have to also take into account that they are sponsored by Steelseries. Other teams that have been sponsored by razer for example, and have been extremely successful with Razer equipment. Some Korean Starcraft 2 players have gone to tournaments with keyboards that people here think of as being the equivalent of a keyboard made out of cardboard (old rubber dome crap) and absolutely dominated.
I think what you need to take away from this is that even at the highest level of competitive gaming, there are different kinds of equipment that are all viable, as long as they pass some level of performance. Also, equipment is not the determining factor. You just need something that will not get in the way. The steelseries mice, the zowie mice in this topic, mice like the Deathadder and abyssus, G400 etc. all satisfy these requirements. This also explains the variation in mice that are used.
Btw the fnatic and SK gaming these guys also all seem to use mostly Steelseries, think Na'vi does as well. The ikari optical and the xai seem especially popular. The starcraft 2 players in team liquid, though less mouse dependent, use all sorts of things. From the Zowie Mico to the Razer Krait.Edited by kazuyamishima - 3/21/13 at 2:56am