In some cases they do cause a problem, especically in the drought stricken SW -- hillsides are entirely denuded of vegetation and what little grass there is is located in the stream bottoms. Guess where the cows end up? It's a battle between the rancher trying to make a living in a time of low cattle prices and drought requiring a lot of supplemental feeding. I don't blame the rancher, but obviously he is trying to get as much out of the natural growing feed as possible. Damned if you do, damned if you don't.
My question to them, Just what did millions of buffalo do to the "natural" flora and funa, streams and rivers???
Not much. The buffalo were free roaming and could travel long distances. When they chewed up most of the grass, they had the freedom to move on; compare that to livestock crossing over the same ground time and time again looking for a blade of grass.
I have a rancher friend who carrys only enough cattle that can be supported by his land and waterholes. He sold quite a few head so that they wouldn't stave due to a lack of grazing. I suspect most ranchers do the same -- they care about the land as well as their herd. But I have seen the opposite too. Polluted streams, and barren hillsides, so I know that abuses do occur.