I mean, would you rather have a $10k build or have your server forcefully shut down after you hit $1k in three days?

One of those things is more important to different types of business. In some situations, any downtime at all is worth thousands per hour. In others, the service staying online is only worth hundreds of dollars a week.

So yes, the solution is as simple as giving the user hard spend caps that they can configure. I'd also set the default limits low for new accounts with a giant, obnoxious, flashing red popover that you cannot dismiss until you configure your limits.

However, this would generate less profit for Amazon et al. They have certainly run this calculation and decided they'd earn more money from careless businesses than they'd gain in goodwill. And we all know that goodwill has zero value to companies at FAANG scale. There's absolutely no chance that they haven't considered this. It's partially implemented and an incredibly obvious solution that everyone has been begging for since cloud computing became a thing. The only reason they haven't implemented this is purely greed and malice.