Doggie in the window

Walking out to get my mail I saw a dog looking out the window of a house down the street. He looked familiar to me and then I remembered where I had seen him. It was in my yard without a leash leaving a message for me in the grass. His owner stood a short ways away and watched him and then left. I thought he could have at least cleaned up the mess. I have two cats that stay inside and don't leave messages in the grass. Everytime I mow my lawn I happen to find notes from all over the neighborhood. Why are some people like that?