It is up to the owner (and their terms and conditions, natch). But it should be opt-in, the same way any other sort of permissive content licensing is explicitly opt-in, and the same way it's bad form to take content that isn't permissively licensed and reuse it on other sites without explicit permission. There's no reason why taking comment content should be treated differently from taking article content.

Hijacking other people's content without permission should not be considered a business model.

And for all the people who have modded me down, why? Explain the flaw in my logic, or is it a case of "Well, I like it and he doesn't so I'll mod him down"?