I think the sandbox idea is a great one. They should just do away with the free tier entirely except for sandbox accounts in which everything just gets shut down the second you go over the free allowance. If you want to build something for real then you pay for whatever resources you use, but if you just want to tinker around and learn a few things then you can get a safe sandbox to do it in.

BUT, I think the parent's point is that such a feature would actually be quite complicated. It's not just a matter of saying "I only want to spend $X in this account per month/total" but defining exactly what you want to do in the case where you hit that limit. Shut everything down? My guess is almost nobody would want to do that. So it ends up being some complicated configuration where you have to deeply understand all of the services and their billing models in order to configure it in the first place. What are the odds that the student who accidentally spins up 100 EC2s for a school project is going to configure this tool correctly?

But I do think the sandbox would be great. Either you are a professional in which case it is your responsibility to manage your system and put in appropriate controls to prevent huge unexpected bills or you are a student (in the general sense of someone learning AWS, not necessarily just someone in school) in which case they provide a safe environment for you to experiment.