It may just be such a basic tenant of the platform that no one thought to. You stop getting billed after lambda returns a response so why would you expect computation to continue? This guy expected free lunch.
I don't see that on the main AWS Lambda pages. It just says that you pay for what you use. It would make sense that the time billed would be until there is no more code to execute.
So many times I've caught myself thinking "I don't want to understand this shit, I just wanna fix it," as I've grudgingly opened up whatever docs I've avoided reading; almost as many times as I've wondered "what fucking moron wrote this code" before immediately `git blame`ing myself.
Fair enough, I guess this just seems like a bold assumption to make since an explicit handler function is a cornerstone of lambda, rather than being able to run module level code and having the end self detected.