I think there is another reason why Rotom is banned too. Because PBR was released way before Platinum, it doesn't include Pokemon such as Skymin, Giratina's origin forme, and the new Rotoms. I believe the finals are hosted on some big screen using PBR, so anyone using a Rotom forme at that point wouldn't be allowed to play. It's a shame though, Rotom formes are awesome and a great help against some of the more popular Pokemon you see in VGS tournaments (such as Metagross and Snorlax).
There was a Dragonite lvl 50 giveaway a while ago, which is why Dragonite isn't on that ban list I suppose (but getting a legit Dragonite for this is probably going to be difficult). They didn't do this for Tyranitar though, which struck me as a bit weird because I thought the green guy would have been the more popular Pokemon.