Not only do some cat owners let their cats run wild, but dog owners do it too. In fact, in my area, I see many more dogs on the side of the road than I do cats. Plus the fact that I live in the country and out here is where everyone thinks it's okay to come and dump their unwanted pets.
I have a stray cat at my house now that someone dumped out there. She is sweet and wants to come in, but I have two dogs that have not been raised around cats and will eat her if I let her in, so out she stays.