Blocking everything except Chrome would immediately raise way too many eyebrows, but allowing the few big (non-) competitors while making life hard for any potential rising stars has value if you want to maintain the status quo (near monopoly on browser market).

In any case, I don't really even care about Google's motivations here. I have a problem with the fact that they can do this at all. Open technology should not enable overriding user control and arbitrarily blocking clients based on prejudice.