You're configuring something that costs money (electricity, hardware, real estate) to provide. Either it's "pay as you go" or you have a flat rate and a cap.
If you have a cap and then your thing hits the front page and suddenly has 10000% more legitimate traffic than usual, and you want the legitimate traffic, they're going to get an error page instead of what you want. If there is no cap, you're going to get a large bill. People hate both of those things and will complain regardless of which one actually happens.
The main thing Google is screwing up here is not giving you the choice between them.
If you have a cap and then your thing hits the front page and suddenly has 10000% more legitimate traffic than usual, and you want the legitimate traffic, they're going to get an error page instead of what you want. If there is no cap, you're going to get a large bill. People hate both of those things and will complain regardless of which one actually happens.
The main thing Google is screwing up here is not giving you the choice between them.