* Amazon has already chosen its HQ2 city, for a variety of business-related factors that have nothing to do with tax incentives.
* But it would like tax incentives if it can get them.
* So it acts like it hasn't made up its mind and gets a bunch of potential suitors to woo it with tax incentives.
If this were a teen movie, the cities would eventually find out that Amazon has been playing them. They would then team up to teach Amazon a lesson, and hilarious hijinks would ensue.
Here in real life, it would be great if all of the finalist cities were to band together and say, "We're withdrawing all tax incentives we previously offered. Choose us for us, not for what tax money we're willing to sacrifice."
But a HUGE part of such a pact is that the cities need to find a way to punish any city that doesn't join the agreement, such as with economic sanctions. I'm not totally sure whether it would be possible to craft sanctions that might deter defecting from the pact, especially with so much potential upside on the line, but that's sort of what has to happen for the pact to work.
One other thing to consider, which is part of the economists' argument, is that having the HQ2 city forgo millions or billions in taxes is ultimately bad for Amazon. HQ2 is going to have employees who will have to live in that city, and it is in their best interests to have well-funded public services.