Dog owners are disgusting if they don't clean up their dogs mess when they take them for walkies.
Dog mess in a public place, sure.
But how much more disgusting are cat owners who let their animals run wild all over the neighbourhood, in the sure and certain knowledge that they'll crap in their neighbours' private gardens and uproot their neighbours' plants? Our immediate area has at least half-a-dozen people with cats. Only one has the good sense to keep it in her flat and backyard - the rest let them run wild. The reason: "It's cruel to keep it cooped up inside." Well how much more cruel is it to inflict that animal's filth on the rest of the neighbourhood?
As far as I'm concerned, if you've got a cat, you either need a good acre of garden or a house completely surrounded by woods or farmland. If you don't have either of those, your cat should only be outside your house when you are and should never leave your property. If your cat can't tolerate that, you shouldn't own the cat.
In other words, it's a matter of responsibility. Cat owners seem to have cats because they think it relieves them of the responsibility of caring for it - it'll entertain itself and crap outside without needing your assistance. Dog owners take on their pets in the knowledge that they'll have to put in some active work with it.