nobody says that the hard limit should be there for everybody, but I SHOULD be able to hit a checkbox that says "no matter what happens, runaway CPU, somebody DDOSing me with traffic, runaway disk, I do not want to be responsible for more than $x/month"

Personally this is the main reason why I have never considered using AWS for my small projects, but maybe this is an intentional choice by Amazon, to keep away "hobbyists" and only go after companies where an extra $1k in AWS bills this month is just a blip on the radar...