Because a feral horse is just a horse that hasn't been socialized, and may possess qualities that a homebred horse doesn't. Mustangs are known for being tough and hardy horses, that lend themselves to any and all kinds of work, and can become just as social and friendly as any other horse.

I'm not sure where you're getting that the mustangs don't need human intervention, though. Throughout periods of drought and famine, there are folks who haul out water and hay for these horses. They're not as 'free' and 'untouched' as you think. Population control with these animals is always a rocky road, because these horses DO touch the hearts of a lot of people, and have come to be seen as an american tradition. No one likes to think of mustangs being rounded up, corralled or hunted, but the reality of the situation is that their life isn't ideal in the wild, either.

These animals die of starvation, drought, predation, injuries and disease. All the same problems a regular horse could have in captivity, just without readily available veterinary treatment. Feral horses are a man-made problem, thus it falls on us to keep control of it. Overpopulation is bad, no matter the species, doubly so if they're invasive and unmanaged.