The article did not mention the teen acting violently, hitting or anything like that. They did mention him running around. If they are not going to let people board who are running around, I'm never getting on another flight with my toddler. He is a tornado in the boarding area, I try my best, but sitting still is not his strong suit and I have a vested interest in letting him wear himself out before the flight.
Just because the kid was running around beforehand does not mean he would be disruptive on the flight. Maybe the parents had things to entertain and distract the teen, but were waiting until they got on the plane to pull them out. I can understand if he was compromising safety on the flight, but before he even boards, that's crazy.