Re: Subordinating Conjuction + Comma?
I can't agree with this reasoning.
I left the party because the babysitter called me to tell me that my child was sick. No comma.
I'd say the comma is more like "So, I'm going to give you some additional information now."
Did you see Peter do that amazing backflip at the party?
No, I had left the party by then [comma - oh, and if you care, it's because the babysitter had called...]
It provides amplifying information, or clarifying information -- the main point of what I'm saying is that I missed the backflip, so the "because" part isn't that essential.
It has more to do with the writer's intent in telling you what is the essential part of the sentence, or the heart of the meaning, and less to do with a strict grammar rule.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.