Building a business on blank cheques and accidental spends is shady. It's also a large barrier to adoption. The more times devs see reports like, "I tried [random 20-minute tutorial] and woke up to a bill for my life's savings and luckily support waived the fee this one time but next time they're coming for my house", the less they'll want to explore your offerings.
Even having the option of a hard spend limit would be hazardous, because accounting teams might push the use of such tools, and thereby risk data loss incidents when problems happen.
Hard spend limits might make sense for indie / SME focused cloud vendors though.
Yada yada yada, that's the same old excuse the cloud providers trot out.
Now, forgive me for my clearly Nobel Prize winning levels of intellect when I point out the following...
Number one: You would not have to turn on the hard spend limit if such functionality were to be provided.
Number two: You could enable customers to set up hard limits IN CONJUNCTION WITH alerts and soft limits, i.e. hitting the hard limit would be the last resort. A bit like trains hitting the buffers at a station ... that is preferable to killing people at the end of the platform. The same with hard spend limits, hitting the limit is better than waking up in the morning to a $1m cloud bill.
Then apply the hard limit in the billing code. If it took a minute or two to shut off all the instances, maybe the customer's bill should have been $1.001M instead of $1M, but cap the bill to $1M anyway. Given their profit margins of x,000% I think they can afford the lost pennies.