Is that a rule?
As I have always been taught, the comma that is present is required. It is a subordinate clause, as it begins with a subordinating conjunction. When a subordinating clause is followed by an independent clause, a comma is required. A comma is not required when the subordinate clause follows the main clause.
The purpose of language is to communicate. If the rules get in the way, fuck 'em.
In my experience, the one and only rule of grammar that you can always count on is that no matter how you use it someone will be along in a minute to tell you you’re doing it wrong.
you’re doing it wrong.
Then there’s the famous Eats, Shoots & Leaves.
I think there’s a federal statute requiring passive voice in tech writing and bureaucratic reports.
Did these teachers think comma usage especially had anything to do with that?
Any linguist, especially one who’s worked in pragmatics, such as myself, would agree that this assertion is probably too simplistic to reflect the full-scope of how language functions.