Amazon as a multi-national, multi-billion dollar company can absorb the expense of employing him. I’m the worst case, they can lay off an existing low performing employee to compensate.
That is a very charitable opinion. The idea that because a company is large enough, or profitable enough, that it therefore should be compelled to hire indefinitely, or never shrink in size.
Let me ask, is it your opinion that economies only ever grow forward, that companies in the economy never shrink?
Training new employees is not a worthwhile investment if you are in the situation where you're already firing thousands.
There are countless other ways they can compensate for the cost of hiring him. A couple quick examples are delaying promoting and giving a raise to some people, or removing another open position they were looking to hire.
The result of not just sucking it it up and doing the right thing here, which was to actually hire him as promised, is 1) a terrible public image affecting future ability to hire talent when people are weighing Amazon vs CompanyX, and 2) increased chance that future new hires will read about this kind of bs behavior and not think twice about accepting an Amazon job offer and then rescinding their promise to start employment, at the very last moment.