Actually, both sides always end up being criticized in the stroller threads.
People, as a whole, have a need to be validated. Whether or not people would readily admit that is a different story. If person A disagrees with person B's way of doing things, person B too often feels that person A is saying that person B is doing something wrong. Person A could just be saying that he does things differently. Person B doesn't feel validated, therefore takes the entire situation personally. When people take situations personally, their nature (on average) is to lash out personally. It ends up becoming an endless cycle, which is why you see threads such as this become so heated.
Those bothered by strollers would say that the child doesn't need to be in the park if he/she can't walk and not throw a fit.
'Tis the DIS...it's the nature of the beast.