I really don't think its just a file integrity issue, although that is the easiest to mention, and the easiest to defeat.

Microsoft has spent untold advertising dollars to inform people that the only legal way to get Microsoft products is from a Microsoft Reseller.  Many non-beta microsoft products are illegally distributed using bittorrents.  To allow this one bittorrent to be legitimate would be taking all that advertising money and flushing it down the toilet.

Maybe I just think like a laywer, (although I am not one) but I am a lot more surprised that Chris Prillio thought that it would be tolerated than that it was eventually shut down.