I think it comes down to them seeing themselves as a utility. You wouldn't want to be prompted for payment confirmation every time you plug something into an outlet, but this does mean that you'll have a huge unexpected charge if you accidentally leave on the air conditioner when you go on vacation.
I think part of the reason that utilities can get away with this is the fact that the maximum bill you are likely to run up is generally 2x-3x your normal bill. This doesn't well for Amazon because your actual bill can be orders of magnitude larger than what you expect.
I'm sure they see it that way. The problem here is that they aren't a utility unless the utility was also selling air conditioners and microwaves, and those air conditioners and microwaves had a button that said "Charge met 10x my normal bill this month" on them.
