Why I don't have kids

I walked to Subway today, and when I entered, I saw a family with a mom, a teenage son, and a smaller son. The little boy was wearing a batman helmet and had a plastic sword that he was beating his brother with. I stepped up to the counter and proceeded to give my order. Next thing I know, the kid is whacking me in the butt with his sword, repeatedly! I told him, "Kid, you really don't want to be doing that." He continued, and giggled while he did so. I told him again, "Kid, if you don't knock it off, you'll be sorry." The whole time the mom just sits there and doesn't stop him.

Just when I'm at the point where I'm going to rip the sword out of his hands and beat him with it, he notices his mom has left him in the shop, and he goes running out the door after her. I turn to the next guy in line, and tell him, "I'm glad that brat's not mine." He agrees with me, and we both go on our way.

What kind of parent lets their kid do that to a perfect stranger, and then leaves him with me?