I had an Acorn Atom (still have it, though I can't get it working any more). Nothing in there suggested 2 spaces after a full stop ;) And the manual used to talk about the "computer programme"!
I guess it's down to teacher and style, as you say. I find it interesting that I never met this until post 2000 internet forums with Americans, though. I wonder how widespread it is.
And like tabs vs spaces, I get that there's a "this is the way" answer. Tabs are the right answer, and anyone who says spaces is an idiot. But 2 spaces after a full stop is clearly the right way, but I'm buggered if I can bring myself to do that ;)