The reason people do this is very obvious - it's because they are responsible, and sorry if this offends the inner sensitivities of anybody on this thread but it's people's right to take their dogs out for a walk, and it's also their responsibility to pick up their poo rather than leave it on the ground for a child to fall into. However, local authorities also have their responsibility to provide litter bins in parks, woods and on the streets. If someone has been completely responsible and picked up their dog poo only to find there is nowhere to dispose of it the only solution is to hang it on a tree, away from the ground where it could be trodden on, and to bring to the attention of the local council the need for a litter bin. People who don't care about their dog poo just leave it on the ground, so next time you see a plastic bag hanging from a tree, approach this with a different perspective, be glad that you are in a neighbourhood where dog owners are responsible - oh and before you say they should take the poo home - it's not practical to carry it around, especially if you have a child and need to hold a hand.