I think context is key. I've seen a fair bit of this "correction" going on in another online venue (a forum in which a lot of high minded arguing of lofty ideas goes on), and it usually indicates that the person doing the correcting doesn't have have a leg to stand on and has had to revert to nit-picking the format and not the substance of their opponent's position.

However, this is not what you're talking about. Each speech community has its own set of norms, and sometimes a gentle nudge to help someone adhere to the group's standards are in order. This does seem to be the job of mods, but an individual could always PM the unknowing offender and thus make the "correction" seem more helpful and less like a public admonishing.

Of course, if you really dislike this person and want to put them in their place in a public way, this goes beyond simply correcting a breech of netiquette.