[toohotatopic]

From google's perspective, I would assume that it is much like MS allowing software pirating: It doesn't matter if you watch the ads as long as you don't watch the ads on youtube.

Google needs your preferences more than they need to show you an ad immediately. They can also show you an ad on their search space or any of the sides that use the Google ad network. It actually might be cheaper for them to show you the ad elsewhere when they don't have to pay the youtube content creator as I would assume that the creators don't get any money if their audience doesn't watch the immediate ads in the video.