Even AWS customers who don't have a contract (like reserved instances or an Enterprise Agreement) may not have any claim to receive ongoing service.
Business disputes are as old as businesses and we have ways to resolve those disputes: the courts.
And be sure to read the fine print of any service or company that you are going to do business with if your business will be dependent on that other party. Avoid such dependencies as much as possible.
So yes, signing up with AWS is a calculated risk. You should know the possible downsides of that decision, and you should be aware of your position, rights and obligations as well as the terms of service.
Most of society actually works like this, and we somehow manage to muddle through without shooting each other most of the time.
If you allow yourself to be locked in to your hosting provider you are doing it wrong.
AWS is very clever about this but it is still your decision on whether or not you allow yourself to be suckered in. I point this out with some regularity to the companies I look at.
Practically, when you have a business dispute with a major corporation, your only option is to give up.
In Europe the courts are still accessible for companies that do not have millions of $.
AWS on the other hand has plenty of competition, as long as you don't willingly lock yourself into their platform you'll be fine.
It's a mall with a large number of people flowing through it rather than just a content distribution engine.