I dislike sharing the wilderness with sloppy horsepackers as much as sharing it with certain irresponsible hunters (similar culture) who basically act as if they have the right to do whatever they like to public land. Now I say that as a hunter. Horse culture might be very influential in certain parts of the country, just as snowmachiners in other regions, but organizations like the Sierra Club, NOLS, localized outdoor groups (outing clubs, Boy Scouts), and even local media outlets can be too, and they could all do a lot more to bring this issue into the light. But only if those like us keep making noise -- and in more audible places than Chaff at BPL.
Every major university, and many community colleges, have an outing club, and a local enviro group. If horsepacking is a problem in your area, drop them a letter about it. You'd be surprised how responsive some of these groups can be, given time. They don't mind being controversial, its good publicity.
Since NOLS has a horsepacking curriculum, I do think they should be at the forefront of political action for stricter regulations in wilderness areas. I'm sure there's a few NOLS alumni on these boards, anyone know a contact? I think a gentleman named Tom Reed used to teach the course. An online petition is a relatively easy thing to organize, and with the backing of NOLS Horsepacking and some rangers from relevant regions, a few local policy makers might look at it.
Where are the problem areas in the U.S.? Lets get a short list going here, of the worst ones.