I am not American, so I can't comment on the issue with your mustangs.
Our brumby problem is at the point where, while some are rounded up via helicopter and shipped to select trainers (there is one about 5 minutes away from me), others are simply shot.
Of course as a horse lover I would much prefer that they could all be brought in, trained and re-homed. That just is not an option. Too many people are full up on rescues already. And the average joe horse owner does not have the knowledge or skills to continue the work with a newly captive horse.
I can not see how round ups are a problem, when over here we are at the point where they have to be culled.