If Google penalized or delisted TripAdvisor, they would go out of business.
Travel companies pay Google for the ads. It's not unusual that the referral costs as much as the margin earned from the customer. Thus the sale can have a zero or even negative value for the travel company, it will only make money from that customer if he comes back later.
Your assumption was misplaced. The good question should be how much money does Google make from TripAdvisor? The answer is that they will lose a lot if they stop referring them.
For instance, if a channel consumes the vast majority of the margin, that's still a valid channel. The advertiser could depend more on you than you depend on him.
It's a very hot topic between Google and the travel groups.