Amazon wants Twitter to spend according to contract; refusing service would run counter that. When there is a contract, there are hefty legal switches one can throw. No need to fiddle with resource knobs.
Cutting service is the very last cudgel to come out for large contracts. And only when counterparty's insolvency is looming. Which may or may not be the case for Twitter.
If Amazon concluded Twitter's insolvency was near it would certainly negotiate on the contract. Before cutting service. Which is why it's interesting to watch Amazon's reaction here.
But I think what you’re saying seems more likely