Hmmm. A lively evening on the Boards here. Must be raining everywhere... .
You're right of course, but it's a level of detailed explanation I just avoided. At the moment, stock are allowed over much of the Sierra, though grazing is becoming increasingly restricted. Technically, we can control those use patterns, but my main point is we can control where people crap and what they do with it. We can't do that with horses and mules, though I also have to say we've never tried... . It's not very equitable (also, a teensy, teensy play on words -- equine -- heh, heh, heh) but it's definitely not sheer hypocrisy because of the relative health risks between humans and horses.
I was initially (two years ago) hugely skeptical and moderately against Wag Bags. However, in their first year on the Whitney Trail, they were getting about 25% compliance. This is truly huge and shows a level of concern that I find very encouraging -- people are willing to do something distasteful to improve their hiking environment. That level of voluntary compliance is enough to bring about a major improvement in the sanitation and esthetics of the Whitney trail area.
Also, in my experience, it's all about education. Sure, there's always going to be people who blow off regulations for whatever reasons (usually it's "why, you young punk ranger, I've been hiking for 30, 40, 50 years and I've never..............."). Still, most people come around. We saw that in picking up trash (vs. burying it); in not building huge firepits; in not building camp tables and posts and nails in trees; and, lately, in bear cannisters. The people who hike are actually pretty intelligent and decent people who want to do the right thing. The right thing changes with time and as we know more about the ecosystems. For years we've been looking for some solution to human waste and Wag Bags look to become a major part of that solution.