I think that might be because some people have made the mistake of getting a steel 'chamber' brush, and running it down the bore, rather than a bronze, or brass bore brush.
A few places that I've been to have the brushes just hanging on a hook, and people don't pay attention. They grab a brush that says .22LR/.223, just as an example, and don't read the label where it says 'Chamber Brush.' I've made that mistake myself, being in a hurry. Fortunately for me, the guy at the counter caught it, "Are you sure you want this one?" kind of thing.
Those flexible bore cleaners have the brass brushes built in to them.