They don't just send you a bill for $10k. You can opt-out of the yearly audit by removing your use of restricted scopes.
But yes, it is a yearly required audit, and they're serious about it. This limits the kind of apps that can be built on Gmail (basically – no free apps), but it is undoubtedly better for end users.
Having gone through the process, which checks among other things that data can't be resold, tokens are encrypted, user data is really deleted when you say it is, and that Gmail API access is auditable in the event of a breach; these are all good things for users.
(My auditor was so thorough they actually found a high-impact XSS bug in Firefox – the bounty covered part of their fees.)