The fact that they are trying to build an ad business can be countered with the fact that they have not yet built an ad business _successfully_.

As such they don't really lose anything by taking away Google's core business in the form of an ad blocker. Microsoft has always been very worried about Google, and Balmer has compared Microsoft and Google as rivals in the software industry. If Microsoft can leverage their control over browsers in IE8 with an ad blocker, then they can potentially take a large chunk out of Google's bottom line, and make Google a much weaker 'rival'.