My opinion on this is still very much in development, so I don't suggest that this is the only right answer.
But here's a potential solution: since API clients are essentially renting the vendor's platform and launching businesses based on this, there needs to be some legally-enforced stability. I would like to see something like a landlord-tenant relationship. Both parties have responsibilities and expectations and the lease can't break the fundamental rules in the law, which usually includes mandatory notice periods to terminate tenancy if a tenant is month-to-month, a prescription for informing the landlord of maintenance issues and giving him time to repair, a legally protected remedy if the landlord fails to repair something that he's legally obligated to maintain (like withholding rent), and finally, an eviction process that the landlord can use to force the tenant out of the property when he's been found in violation of the lease.
I think this sort of relationship should exist at a minimum. It would make it so Google et al couldn't just kick you off whenever they got bored and decided your product was cool and they didn't have anything to do with their 20% time so they're gonna clone it, they could only cut you off at the conclusion of your contract. If there's a legitimate reason to kick someone off the API before this time, they can take it to an API tenancy court and get the judge to sign off on the access key's eviction (which should be thousands of times more practical for both parties than the current route, which is a CFAA lawsuit).
The ability to "buy land" on the platform and own an API key that can't be deprived from you would be interesting too, but I'm not sure on the details of how that would work right now.