Nope, people can and do improve the land while feral animals think only of themselves and destroy it, along with themselves and the native species.
I have no problem if they want to wipe out the mustang herds completely. It's not like horses are endangered, unlike some of the native species whose habitats they're destroying.
As far as feral horses being 'living history', that's all just a bunch of sentimental claptrap. Why no tears and hand wringing for all the oxen who were the majority of animals used for moving wagons and plowing for all those settlers moving west? Oh that's right, they were used up then eaten, but that's okay 'cause they were just stupid cattle.
Cattle - specifically bison and oxen - are HUGELY beneficial to prairies and big fields of grasses. We just learned about this in my environmental science management class - they are a keystone species (along with Prairie Dogs) to the well being of grasslands. While a herd of horses will eat the ground down to dirt and tear up the soil, a herd of bison or oxen eat selectively and their rooting around is good for the soil. (I forgot why though. Shows how much I pay attention).
I'm not sure how I feel about the whole mustang situation. I admit, I don't know a lot about it, so my understanding of it may not be correct. I don't think a herd of feral horses should be allowed to destroy grassland, eat it down, need to be fed and watered, then round up and sold. It's an awful life and it's terrible for the land.
I think that most of them should be rounded up and done away with - that many horses could feed an awful lot of people (sadly not here in the U.S.) or make plenty of dogs happy. Horses are not an endangered species. Bison and wolves are. I guess I just don't see a need for SO MANY mustangs. I think, if people really want mustangs, there should be designated areas for them. Huge areas, licensed, that are capable of holding that many horses without destroying the land, and that round up the horses with horse and rider. But I guess I genuinely don't see a need - stop protecting them. If they can't survive on their own they're just eating resources. Let them REALLY go free and see how long they last, with the wolves, bears, native animals, and no ranchers to feed and water them. I don't see a problem with them if they can make it on their own without destroying the land.
...Again. My knowledge of the situation is limited, so I'm sorry if I got any details wrong :p