The bags I've seen have been left, and have been there for a time – no intention to pick back up at the end of walk.
I've seen these bags hung all over the place where others have to pass, thus upsetting others.
I would really like to find out the reasoning behind why they leave them behind. They don't get picked up, and the only proper action is to dispose of it, correctly if you have picked it up in a bag.
When I am walking my dog in a wood or on the hills and she has has an evacuation, I kick it into the side making sure no-one will step in it.
Couple of days later nature will take its course and will be return back to the earth