It may be. But it is true that most tax havens, including the other three I mentioned, do not tax anything that does not happen within their borders. That is why there is the infamous address in the Caymans to which some 8000 businesses are registered, according to Obama.
"Delaware LLCs pay a $100 annual tax and are otherwise tax free if no business activity is carried on in the US, most of its members are not US citizens or residents and it does not invest in US companies."
That's a lot of if's that there's no way a company like Amazon could satisfy. Also:
"Although there is no state income tax, as with all states no matter their particular tax laws, all US companies (or any entity for that matter) are subject to applicable federal tax laws & reporting requirements."
In other words, of course US taxes are still payable for companies incorporated in Delaware.