Earlier quoted context omitted.
I've experienced plenty of scenarios where costs have quite legitimately spiked. Ultimately whatever solution you put in place, someone is going to complain about it. At least with the system they currently have in place they can reimburse customers. Whereas it is a lot harder to fix their reputation after they've automatically stopped production services.
Given how easily they reimburse customers, I suspect it's intentional - one can be "fixed after the fact" and the other can't - if your site goes down during a slashdotting and you lose sales, etc, there's no getting those back, but if you inadvertently run costs high, they can just refund/cancel those costs.
Ive seen nobody on HN, twitter, reddit complain about "my site was down during heavy business since i turned on hard billing setting". Not a single person.
However, I see frantic after frantic post of "I was testing something on AWS and it caused me a $X000 or $X0000 bill."
But as the posts in here are apt to suggest - you can always beg AWS support for a reversal. Great plan there.