"Legal" doesn't necessarily imply "ethical" -- interracial and homosexual relationships come to mind, both of which were illegal within recent memory. Thus, it's not enough that megacorps seeking tax havens "follow the law" when they have such resources as to ensure the law comports to their desires. It seems "proportionate" to want those with disproportionate power to have a little less...
>Also, surely most giant corporations are not legally in the Cayman islands, but Delaware, right?
Ugland House [0] is the building I'm describing. Despite being five stories tall, it's the registered office address for 40,000 US firms. You're certainly not wrong to invoke Delaware, whose Corporation Trust Center [1], a "suburban law firm"-looking building of modest size, sports seven times the number of fake corporate headquarters as does Ugland House. I'm given to understand that there are various benefits to the Caymans' regulatory environment, and those that don't have to be on US soil tend to prefer it.
[0] https://en.wikipedia.org/wiki/Ugland_House
[1] https://en.wikipedia.org/wiki/Corporation_Trust_Center_(CT_C...
It sounds like you are in favor of that, but then your first two examples (Amazon vs the Caymans) are confusing - what is so palpably evil about the latter? Also, surely most giant corporations are not legally in the Cayman islands, but Delaware, right?