I agree, actually, and I use it exactly in the way you mention. And mostly, I don't get many, if any, comments like the one my friend got, so apparently I'm doing a good job of warning away people who want to be warned away?

I haven't run into any mislabeled fanworks with such an extreme warnings issue before, so I'm sorry that happened to you. And yes, I would probably get my nose out of joint, too - it's one thing to choose not to warn and announce that fact, it's entirely different to choose not to warn and then actively deceive the reader that there's nothing in your story that might be disturbing or even harmful.