The tenants are behaving like this because it's not their property and they don't give a damn.
Dog faeces cannot be used as manure and as they sound like irresponsible dog owners, (a responsible one would clean up after the dog). Not only is the dog walking in the mess when it's going out, it's probably bringing it back into the house and all over the furniture.
My partner's mother has 3 dogs and he goes there everyday, scoops the dog mess up and puts it into a bucket of Jeyes disinfectant solution, which breaks it down after a couple of days, it then gets poured down the drain. He also hoses the patio area down everyday, otherwise it would stink and be unhealthy for both the dog and it's owners.
As they sound irresponsible, they possibly aren't worming the dog, so the dog faeces could contain Toxicara, which can cause blindness to children who come into contact with it. I would kick these tenants into touch, they are disrespecting his property and taking him for a fool.