I grudgingly admit that it’s a plausible-sounding rule, and in the textbooks and stuff. But, to be honest, I read it too many times in various posts and comments threads the other day, and in my raging pique I found myself thinking that the next time it happened I would say, "That’s completely backwards: in fact, causation is just correlation" and fling a copy of Hume’s first Enquiry at their head. Or at the screen, I suppose, but that image is less satisfying, because now who’s the crank on the internet, etc.
I know the feeling. I would settle for asking "Why not?" because what it is that makes causation different from correlation is always swept under the rug; and although I'm pretty sure Hume's wrong, I think that if some common assumptions about causes were true Hume would have to be right (from which follows by modus tollens, etc.). In any case, uncritical complacency about a slogan is more than a bit annoying, even when the slogan is right.
But to do it properly you'd need to fling it repeatedly at the back of their head.