I found "Treasure Islands: Tax Havens and the Men who Stole the World" a very interesting book on tax havens, and it made the point about the US.

I also found this idea from the book enlighting: All the tax havens around the US, they don't have 'real' banks, they put the money into NYC banks. Same with the UK: Tax havens in the cloud/former empire of the UK put the money into banks in London. The real beneficiary are not those islands, but US and UK banks.