A few years ago, our local council decided that it would be a good idea to rip out the dozen or so bins scattered around my local park and replace them with four slightly larger ones at the entrances. That included removing the three 'dog waste only' bins at the enclosed dog run. Prior to the bins being replaced, hanging bags of poo were non-existent, you were never more than a couple of minutes walk away from a bin. A week after the replacement all the bins at the park were full to overflowing and so where the ones at the nearby shop. The trees and fences had disgusting new fruit hanging from them and the local FB group was blowing up with people complaining about dog walkers using their wheelie bins. I only lived a five minute walk away so I could drop my dog's bagged shit in my own wheelie bin, but a lot of walkers who used the park drove there (because it was one of the only ones with an enclosed dog run).

I don't condone them leaving their shit behind, but I can understand them not wanting to have to share a car with stinking bag of shit and I'm guessing that at least part of the reasoning behind leaving the bags hanging on a fence or bush is that it was a) a visible protest about the council's idiocy and b) if the bags are hanging, they pose slightly less of a hazard to the wildlife and can be more easily seen and collected.