Whitefeather--While that was an sort of subsidiary part of the justification, the Park's primary contention was an outright falsehood; namely, that the backcountry was overcrowded. Their own campsite reservation data readily disproved that claim, and they further compromised their position, at least in my view, by basing this on apparently undocumented "complaints" as opposed to hard evidence observed by rangers. The simple truth is that rangers of today seldom venture into the backcountry, a sharp contrast with those of yesteryear.
As for messy campsites, it has been my experience, stretching over a lot of years in the Park, that when you see a really messy campsite it almost invariably is linked with the horse crowd. I'm not condemning all of those who use horses in the backcountry, but I do firmly believe that two things are obvious: (1) Horse camps are messier, smellier, and generally less appealing that others and (2) Horses do a great deal of environmental damage to trails, plants, and in general.
However, the horse "lobby" is apparently a powerful one, because I've written to four or five superintendents over the years on this issue and invariably get a response couched in "they use the Park too" terms without any response whatsoever to environmental concerns.