I think the vast majority of threads here involve trying to "correct someone's rudeness," whether by pointing it out to them directly or changing one's own behavior so there's no opportunity for the rudeness to occur. E.g., you can tell someone that their lateness inconveniences you, or you can stop inviting them to things where promptness matters. The former is more direct and straight-forward, the latter more subtle but potentially with less drama. Though with the latter you may end up explaining your reason anyway, if the person finally asks you why they don't get invited to stuff anymore.
I think "rude to point out rudeness" involves situations where the "rudeness" doesn't really affect you personally, where calling someone "rude" is actually just name-calling so you can feel superior. Or you're sticking your nose in where it doesn't belong. Like if you said to a stranger in front of you in the grocery check-out lane, "Excuse me, could you not talk on your cell phone right now? It's rude to the cashier because she might need to ask you something and you aren't paying attention to her." The wording in itself is polite and you're explaining the reason why something is bad (which I think is useful). But if someone posted here that they said this, I think a lot of people would go, "You were rude to point out her rudeness! You should have minded your own business, it's patronizing to try and school another adult like that." Especially doing it on behalf of the cashier, who presumably could speak up on her own if she felt it appropriate.