You are the second or third person to say Mustangs are feral, not wild. Not trying to be a wise guy here, but do you know what feral is? If not, it is wild.
Wild is wild, whether never domesticated or feral. Feral simply refers to animals descended from animals that were domesticated.
I also respectfully disagree that they "do better when reunited with the species that initially brought them here". On what do you base that? While it is true that parasite control lengthens the natural lifespan of all horses, the same could be said of any grazing animal. Personally, I am of the opinion that Mustangs, as with all wild animals, are most content being left alone.
That doesn't mean I oppose capturing and taming Mustangs, or oppose people that own them. But it is, in my opinion, rather far fetched to say an animal is better off confined and used as a beast of burden than running wild to pursue its instincts, which is what brings it contentment...
There is a difference between a wild animal and a feral one (ie, one that is descended from domesticated animals). Perhaps you would prefer that I use a different word, but the fact is that American mustangs are Man's creation gone untended. We have tampered with their conformation, their temperament, etc. The majority of mustangs can be returned to a domestic lifestyle with relatively little difficulty, and they can be trusted as partners to their humans. If you compare this to something wild
like a zebra raised in captivity, it is nigh on impossible to train them to ride or even to behave in a way that is predictable. They do not develop a reliable connection with the humans working with them and handling them.
I believe that it is far fetched to believe that horses these days are treated as "beasts of burden". They have become a hobby, a pet for the vast majority of people who own them. Special care is taken for grooming, farrier work, nutrition, medical care, etc.
Everyone is entitled to their own opinions. I don't think that all of the mustangs should be rounded up and sold off... but they have no more "right" to the American land than settlers who drove out Native Americans. Just as we competed with and drove out the native peoples of this country, mustangs compete with and drive out the native species of this country. It is important to bear this in mind.