Stupidity: There is no way Mr. Mertyle did not hear the fence in his backyard crash to the ground; there's also a likely chance he could hear his dog crying and Smalls saying the dog is hurt. Even if he could ignore the sounds of other attempts to try to get the ball back; with the fence coming down where his dog lives and the chances of an injured pet it seems far too unlikely he wouldn't have rushed straight to the backyard hearing this.