Grannyknot--Interestingly, over the years, I have gotten different explanations as Park officials attempted to justify allowing extensive use of trails by horses. I've heard the explanation you offer, one to the effect that locals had traditionally used horses (only marginally valid but there's some truth to it), and even suggestions they did no more damage than humans (which is unmitigated hogwash).
One explanation which was never offered, but there's appreciable truth in it, is that no one in a position of authority has had the intestinal fortitude to stand up to a pretty powerful lobby which includes some political bigwigs (same sort of situation which left "summer retreats" for the rich and influential intact, let politically powerful folks stay at the Calhoun House on Hazel Creek, and the like). Another thing which frosts my grits is the tremendous amount of noise pollution from motorcycles. There are places you can be five or six miles from a roadway and hear them.
In truth, it's a balancing act for bureaucrats, but they have long turned a blind eye to the deleterious impact of horses.