Says there is no way 20 cities will agree not to give incentives. It would be hard for two to agree on that and not go back on their word, in fact.
Plus if they really did so, Amazon could and probably would say "turns out none of our finalists met our criteria, so we are considering the additional cities in the following list". It would be a "lesson" to them to provide the giveaways corporate America thinks it is entitled to, and insure the next crop wouldn't make such an agreement.
The only way around this is to pass laws banning the practice, and it would have to be done at a national level otherwise it would be in the interest of states to drag their feet on doing so to give them an advantage over states that went first.
This ought to be a position all conservatives support, if they are true to their position about being against "welfare" and government interference in private markets. Funny how so few of them speak up about this...