Can't blame them. For one thing, they can't guarantee the quality and content of the torrent files, so if they allowed them to stay up without taking some sort of action, they could be liable when somebody caught a virus, etc.
Doesn't mean they have to find every Vista torrent and shut it down, but they need to publicly make a stand on higher profile trackers and publicly state that Vista Beta 2 via torrents are not supported and not legal.
That way, if somebody downloaded an infested or broken version and felt like suing for damages, MSFT would be able to point out that they took steps to stop the 3rd party hosting, and didn't support it. Case dismissed.