I don't think it matters whether Amazon or the city/state engaged the third party - the personal career/campaign finance incentives of the individual politicians/functionaries who negotiate for the city/state are aligned with Amazon's wishes and against those of the taxpayers.

And the third party's incentives are to please the paying customer in showing a good return on investment.

But if the subsidies won't actually get paid unless Amazon delivers on their promises, I agree this deal is way better than most of its type.