Because everyone knows the best way to calm a grazing animal down is to attack it and cause it searing agony. Besides, they didn't want to shoot it, y'see, and being near a farm they couldn't, like, borrow a rope, or slow and direct traffic for 20 minutes while they got somebody out there who knows what the hell he's doing to help round it up... No, instead it's we're The Only Ones "Git Along Little Dogie" Enough...