Yes, that's a good point and I agree to some extent, but there's a line between putting forward valid constructive criticism and just posting an opinion because you want to make a personal point or a moral judgement, which may not be at all constructive to the subject at hand.
In your example of the poo photo (good one

) telling the photographer that poo isn't a very good subject is perfectly valid criticism, because chances are that most people will feel the same way as you and so the photographer will move on to taking pics of more pleasing subjects, which will reach a larger potential audience. So you've helped him/her in a way.
However, in this case it's quite obvious that many many people....most men in fact, are rather partial to photos of attractive women and always have been, so your criticism of the subject matter is totally non-constructive, because you're not only going against the grain of mass opinion, but you won't make the photographer change the subject matter either.....you haven't helped anybody with your criticism!
For example, landscape pics mostly just don't do anything for me at all. No matter how well they're shot or how technically competent they are I just think most of them are boring....that's just me. So if I see a thread with a landscape photo in it I just don't comment, unless I can say something constructive, because I know that many people do like landscape photos and no matter what I say people aren't going to stop taking them.
It's the difference between genuine constructive criticism and just being negative about something because it's not to your personal taste. Your initial post just came across to me as the latter rather than the former.
You're perfectly entitled to your opinion of course, but one thing I've learned about internet discussion forums (fora?) is that if you post your opinion then expect somebody to occasionally challenge it.
Besides, if everybody always agreed with everybody else then places like this would be really boring and nobody would learn anything.