Agreed, I've had large bills for cloud providers, forgetting to terminate a GPU instance, or didn't realize that having a disk image (even not running) costs money.

>> why anyone would sign up

It happens more often than you think: people sign up for credit cards and forget to pay the monthly bill in full. Sign up for a cell phone plan and get charged with large bills of international roaming. People sign up for monthly subscriptions, and exceed the usage limits.