Yeah, there's really two things here. 1) Shutting down the service entirely; 2) shutting off _your_ account in particular. Or really, also 3) Changing the terms (for instance deciding it's not really 'unlimited' after all).
You're talking about 1, and sure, there's not a lot you can do.
As far as 2 and 3, back in the day, when you paid for a service you might sign a contract, and they could not simply decide to cancel your service in the middle of the contract because they didn't like you, for reasons not covered by the contract.
Of course, that also assumes that they wouldn't just put "and by the way we can cancel your service at any time for any reason including just because we don't like you" in the contract, or that you would have some kind of negotiating power to demand different terms or go with a competitor with better terms. But obviously they _would_ just put that in the contract too, as they did in the ToS which is theoretically a contract too.
And there's really another thing too specific to the Amazon terms posted above. They are advertising the service as 'unlimited', but then the terms say they can shut you off if your use "substantially exceeds or differs from normal use by other users." That doesn't really sound "unlimited" exactly, does it? I don't know if putting "Ha ha not really unlimited" in the ToS would protect them from such things as this: https://www.ftc.gov/news-events/press-releases/2014/10/ftc-s...