Well, it's in the thread title, is it not?
Not acceptable on a neighbour's property, not acceptable on an adjacent footpath. There is a regular route to and from a local primary school past my back gate and several decades ago it seemed to be perfectly acceptable to let your beloved pooch squeeze one out wherever it pleased. No doubt it's delightful for the dog, and for some owners their pet can do no wrong. "Aah bless, he's having a dump. Who's a clever boy then".
You can imagine the mess when small children walked to school with their parent, small careless feet landing in the ordure and tracking it for yards along the pavement for other feet to pick up and redistribute. No doubt they learned very quickly to detect suspicious packages on the path and navigate the poopiferous obstacle course. Despite many more kids going to school by car nowadays there's still a lot of foot traffic. Now that dog owners have been "house trained" to collect the whiffy dollops in bags it's a lot more pleasant for the rest of us. Just because most of it's in a bag doesn't mean there isn't some noxious residue left behind. Maybe some dog owners are quite happy to have their own lawns decorated with the pointy ended reminders of thir dogs' previous poops in various stages of decomposition, but it's not something that appeals to most.
The usual response to cats using your garden as a toilet seems to be, get a cat! Since cats will use someone else's garden, and other cats will keep away, it just tends to move the problem elsewhere, rather than solve it..