Because that might be very quick. Maybe you've got a 100x sign up boost in your SaaS app and you're missing out on the profit of your live?
Also, what do you want to shut down? EC2 instances, which might loose you data? Maybe no new instances, but your SaaS app needs those? Shut down all traffic? Delete your S3 files? All of those could be the possible reason for exploding cost and suddenly cutting any of those could be fatal.