I knew about the round-ups, but I thought that the mustangs were sold. In fact, I know that there are/were Mustang Sales, as there were stipulations about their housing, transport, etc.
If, in order to keep populations in check, they are culling the herd, they should open up hunting. I'm sure that folks would hunt them and also sure folks would eat them.
Horses are not native to North America. That means they compete with the native wildlife, much like wild hogs.