One issue is what gets shut down when you hit the max? If you have an EC2 instance running and you hit the max, do they shut it off? Would customers understand that and be ok with it? What if you have an S3 bucket? Should they just delete the data? That's probably not what you want.

You can basically do max bill now -- you can set up a cloud watch alarm for billing and when it reaches a certain price, run a script. Your script could just shut everything down and delete everything, or do whatever is appropriate for your account. That's their solution to this.

Also they don't have instant feedback on usage -> cost. They batch process it. So if you get a huge spike in usage, AWS may not even know that for a while. They could in theory be willing to eat the cost of usage between it happening and their processing, but are probably unwilling.