In this thread, runner pat, doubtless with good intentions, posted to say that he reported the thread as spam. Intentions aside, it struck me as smacking of the sort of officious rudeness you might find in an elementary school hall monitor. Maybe the self-appointed helpers could bear that in mind, especially since it prompted a response that got the OP warned by a moderator.
It prevents others from reporting the same thread again when it really is spam. I think the mods all get an email each time it gets reported. Anyway, its a custom practiced by many, and considered to be the polite thing to do if you are reporting a thread as spam.
The point of mentioning “reported” in spam threads is so that other posters seeing it will know it’s already been called to the mods’ attention. That way the mods don’t get hit with 20 reports for the same thread.
It was an honest mistake, but he wasn’t doing it to be officious.
I’ve posted “reported” many a time on a spam thread. It’s usually considered good form. I’m sure it was meant that way, even if it wasn’t actually spam.
Eta: the OP didn’t get warned. It was just a mod note. “no warning issued”.
Where is manhattan when you really need him?
How about this one, which again prompted an immoderate response, and moderator comment?
I can grasp the usefulness of the practice, but I think it’s being misused a little.
I think that’s a different situation. Most “reported” posts I see are about spam, and that’s fine. Posting “reported” for other rules violations, IMHO, is snarky. if it’s an issue, report it, and the mods will comment on it. No need to post about it.
Yeah, those are different posters and different situations. Reporting obvious (or often obvious) spam is considered polite but noting a perceived rules violation by an authentic poster is usually junior modding.
Officious rudeness? I realize that plenty of sites (far too many, in fact) are basically kindergarten. Luckily for us, the SDMB is not part of that inanity. Adults gather here, so to speak, and interact in an adult manner. “Fear of Snitching” is a childish viewpoint and contributes to the decay of many things that were once good, IMHO. As others have posted above, notifying others who see the spam post about it being reported serves to save those others time and effort; it also serves to save the mods/admins from wading through tons of reports on the same thing.
Well, color me embarrassed. I thought it was considered helpful to post one had reported a thread for any issue, including “wrong forum”. Thought I was helping the mods and other posters. Had no idea it was being perceived as snarky. Ooops…won’t be doing that again. 
(dang fool noobz, always getting things wrong)
I’d only use “Reported” by itself for obvious spam. For something like a wrong forum, I’d post something like, “I think this should be in Great Debates, and I’ve reported it to the mods.” That makes it clear what you are reporting, and isn’t rude like a bare “Reported” would be. For something like an insult or an off-topic posting, I’d just report to the mods, and let them make the judgment. I suspect that in cases like that they won’t get lots of reports anyway.
Reported.
There’s one in every crowd 
I only post “Reported” for obvious or apparent spam if no one else has already done so. I agree that posting “Reported” for any other reason could be considered junior modding, so I just quietly report other posts and don’t make an announcement about it. I think in the first instance, runner pat announced the report with good intentions, as you admit, and honestly thought the thread was spam. As for the OP’s insult, that wasn’t pat’s fault. The OP bore responsibility for receiving the mod note.
Besides, it was six months ago, and it’s just now bothering you?
Would be interesting to hear from the Mods- would they prefer a number of notifications or one with a “reported” note in a thread?
(BTW this is not the first thread on this topic).
Retorted.
Wow, you’re right. I checked the linked thread, saw that the OP’s join date was “Feb 2011”, and thought that maybe we can give him a little slack, as it’s unlikely that a poster who joined seven months ago would be posting a spam thread. But turns out the thread was started in the early hours of March 1st…
Just me personally–“reported” on clear(or suspected) spam threads is OK. Helpful even.
Reporting a forum change or anything else–hit the Report Post/triangle thingy.
Resorted
Deported