Wait... I finally get it. I finally get why so many companies keep going on these boneheaded lawsuits even though they will eventually lose and it waste their time and money.
The reason they do it is BECAUSE they will waste time and money. Or rather, their lawyers tell them to go ahead with these ridiculous lawsuits because they (more than likely) have a contract that says they get paid per hour regardless of whether or not the lawsuit wins or loses, and so it is profitable for them to just tell their client to go ahead with every lawsuit.
There certainly isn't anything 'creative', though copyright seems to have multiple standards today. Either way, legitimate or not, Microsoft used the quickest and easiest tool they had to take down the product keys.
They could have simply invalidated those keys, though I think they may have already been registered by a legitimate user. Simply disabling them in that case could actually land them in hot water. In some places, such as Europe, it is illegal to disable a product that someone has already paid for.
Actually, what I had heard was radically different. I had spoken to a former Microsoft employee who I know, and he stated that what Microsoft had actually made the copyright claims against were the individual comments, and that google/youtube had screwed up by taking down the videos instead.
You know, I think I realized where the support for secondary liability comes from. It comes from the general belief that when you're in charge of something, you are responsible for the actions of your subordinates.
Except, that doesn't really apply in these situations because the users are NOT subordinates. They are entirely outside of the organization, and therefor the people in charge aren't responsible for their actions.