This is my new pet peeve, especially when it's at all programming-related. Anyone dealing with code should know readability is a big deal. I can parse a sentence better with that extra comma without having to reread any part of it. I don't see why its ever left out.
It's an optional feature of the language. Sometimes its exclusion does lead to ambiguity, but other than that, it's a stylistic choice, and preferences on usage diverge.