jerf's explanation makes sense. The negative certainly does not reinforce the positive.

Yes. They said it better. It certainly doesn't reinforce the positive, yet people seem to insist on using it as if it does. "Well, yes, that penny did land on it's edge but the exception proves the rule it had to be either heads or tails" type statements abound. No.. the exception proved pennies have three states not two: your argument is busted.

Exception proves the rule comes from Roman law: if there is a sign saying you can’t park here on Saturdays, it means that you can park here on any other day.

I was taught it comes from logic, and argumentation: the act of stating an exception proves a rule as stated (a rule is a claim to an axiom) is wrong. So it's in logic and philosophy, not Roman law.

If you’re reading a guide book and it specifically says: “Use spell X to ability kill this boss” the implication is you can’t instantly kill other bosses by using ability X.

Logically the statement says nothing beyond what it says, but implicitly by providing the exception you’re stating that the general rule is different.

It’s a perfectly rational linguistic argument, though often misused outside of that context.