Or, “supplying empty bags at the trailhead means picking up full ones along the trail.” Sorry, no picture.
In the last few years, hikers in the Mt. Whitney zone have been issued “wag bags” with their permits, and are required to carry them on the trail. The Whitney zone simply had too many (inexperienced) people leaving turds and toilet paper for the normal backcountry policy of digging catholes to work. However, the wag bag program has hardly been a success: now, instead of finding little surprises behind bushes, you find full bags right along the trail. Someone still needs to clean up, but now the disgusting problem is right next to the trail, and not at all biodegradable.
The Boulder County open space around the flatirons has a similar problem, but with dogs instead of humans. They supply empty bags at the trailheads, and you find full ones all along the trails. Picking up after your dog works in city parks, but even in highly policed and eco-friendly Boulder, it doesn’t work in the wilderness.
I can only conclude that it is futile to ask people to carry bags of poop in the wilderness. It works in a park, where there is always someone watching, but our revulsion at the sight or smell of feces is deeply ingrained. If you ask a bunch of people to do something against which their very lizard-brains rebel, some of them just won’t.