Everyone does remember that google makes their money based on Ad revenue. They have arguably the most visited page in the world which is prime real estate that they generally leave completely white. I think it's a great idea, regardless of how people speculate their device sales are going, to put a unobtrusive ad at the bottom. Sure it catches your eye but you don't have to click an X to get it to go away to use the page or anything like that. I say, good idea.
I should say I'm a proud Nexus 7 owner but if it were some other device Google had a significant interest in, I would feel the same way.