It's "unethical" in Google's world where everybody[1] is trying to play by a fair set of rules. But taking a step back, it is really no more unethical than any other paid endorsement.

[1] Google excepts itself from time to time apparently, so I speculate that it also excepts others- probably for reasons that ultimately benefit G itself. So I'm alert to the idea that G's world may not be all that ethical in this particular department.