The answer is: market share. If Android ends up taking Symbian's market share in 3-4 years, then Google's stock is going to benefit because of people confidence in the Google trade mark. Also, in such a case, 30% of all revenue for all paid applications go to them, which would be a lot of money at that time.
Q. What is Android’s business model?
A. Somebody could rip out the Google stuff and put in Yahoo stuff. That’s ok. Our job is to continue to create killer apps that people will want to use. Google search, GMail, maps, etc.. If we ever fail to delight users our core business will go away. That’s why we felt comfortable using the Apache license.
While we’re showing demos with Google applications, but there is no requirement to use them.