The amount of work Google have done to get code back into the kernel mainline shows how little they like having a fork.
My own experience maintaining just a few small patches for a few small packages encourages me to believe it doesn't get any easier with thousands of patches.
Google are just people. Thinking of them as evil personified with greed on top, the kind of people who laugh in the developers' faces about all the extra work they create for them ... well, it's just silly. If nothing else, think about it from the greedy point of view that it's better to get patches back into mainline and lower dev costs and increase product stability.