Sometimes it really comes off as scolding, condescending and unwelcoming, especially if someone posts merely that one sentence: "I don't see an etiquette question here" or, even worse: "I thought this was an etiquette forum."
If someone actually posts a longer post, trying to be helpful, but at the same time adding "but in my opinion, this is not really an etiquette matter, but a matter of xyz"; or for example "I think this goes beyond etiquette, this is violent behaviour and you don't have to put up with it!" then I think it's good. A poster can express their doubts about something being an etiquette matter without being totally dismissive of the original poster of a topic.
I think, if you have really *nothing* nice to say, don't post. If you have something nice to say + a critical remark that you can make in a polite way, that's fine.