Because Nintendo claims that the images, videos and all other stuff of B&W are copyrighted so they don't allow other sites to redistribute them. Everyone is getting angry about that because they claim that we're all actually advertising for their game so they should be happy and let us do it, but I think that they're scared people start downloading ROMs and stuff because when people read spoilers and news from new games like that they are usually encouraged or tempted to download illegal stuff like ROMs. That's just what I think, I don't know if that's how Nintendo sees it though.