A lot of people that post a picture of a bull get defensive when someone points out that their darling is not up to snuff or that maybe their management of the animal needs a little help.
In the case that you are talking about, the person had a preconcieved notion of how they gonna manage their cattle. (grass, hay and occasional supplements I believe they said.) They bought a bull that had been fed extensively and left him to run with the cows and grow at the same time. He had obviously melted a long time ago. When I and others tried to point out that the cows in the background of the picture were thin looking as well, the person said they were not thin, they just looked like it in the picture. They also were having a problem with cows not cycling until 5 months after calving. They did not want to hear what people had to say about their cattle, because it was pointing out the flaws in their plan, which was to feed cattle incapable of existing solely on grass.... grass. They took their ball and went home. Do you suppose that the guy that sold them the bull in the first place asked them anything about what they were going to do with him or what their environment was? Or did he just cash their check.
I have worked in the purebred industry and know that some of them tend to be on the crooked side. They get a hold of somebody that doesn't know much and sell them what they don't need. Not knowing any better they proudly post a picture and are shocked at the response.
I agree that simply saying that the bull is crap is not the best way to respond, but others usually have pointed out the major points already and the crap poster, (so to speak) is agreeing with what has already been said. I have not seen the first poster say a bull is crap.
If the object of posting a picture is to learn something then I don't think that censuring responses will be very productive. These people that are on here have no financial incentive in lying, they speak the truth or at least their opinion. You don't usually get that from someone trying to sell you something.
Maybe everyone should say: Nice bull? Or do what your mom always said, if you don't have anything nice to say, don't say anything. Not much to be learned from that though.