It is a form of coherence for the system, though. And in the spirit of the same ideas. That is, it is an intentional policy for databases, too.

And there is no "exactly right amount" that makes it work. They keep options for forcing people off flights if they planned it wrong.

Is also why they don't let gate agents over sell a flight. They keep a stronger consistency on that, for this exact reason. Over selling would fit in what someone else called external consistency.

I would wager, in complete ignorance of the real implementation, that the knowledge that they can overbook means that they can relax some of their requirements. If two servers can’t talk to each other to coordinate for a while, they could still each sell tickets.

No, it's because of CAP forces to make a compromise. If there was a way to be available and consistent there would not be overbooking. But they chose availability because losing sales is worse than dealing with overbooking.

Exactly same with Amazon, they will not bother checking that the product is actually available because then you will miss the sale when the warehouse service is down or slow, which it would be all the time because it has to be a single place of failure. Its better to refund orders for non existent items than to miss sales.

Consistency requires single source of truth which implies single place of failure. There is tremendous cost to it, it's not done for no reason like you say.