[Disclaimer - I work on Windows Azure which could be considered a competitor to GAE]

You always have critical bugs in large systems. Unless it is causing an outage or impacting some critical scenario in a major way, you don't want to mess up your usual process. You want to do orderly upgrades to all your environments/clusters/DCs and short-cut/hot patch only in dire emergencies.

Also, it is hard to judge the severity of bugs, especially in large scale distributed systems.