The Core of android is open source, but google charges manufacturers if they want to include certain closed source apps in the ROM, like Gmail, Maps and the Market.
Manufacturers are free to not include those google apps, and write their own alternatives, and indeed the budget ones do, but the official apps are a major selling point that consumers are expecting.
However google main business is advertising. The more time people spend using devices with googles ads in front of them, the more demand there is for their services and the more money they can make. The angry birds guy reckons he'll make more money from his cut of the adverts than from sales, so imagine how much they are worth to google.
Mobile devices also allow google to offer more attractive packages to advertisers, you can be targeted more accurately by location thanks to the GPS (can turn off the service, but I expect most don't bother) The apps store is another great opportunity for google to target advertise you, since now they know what kind of apps you tend to use etc so they can refine their profile of you.
I make it sound like a bad thing, but tbh, If I have to look at adverts I'd rather they be something i'm interested in.