Someone who shows the ability to cause trouble on the ground shows the ability to do so in the air where the options to deal with it become much more limited.
Reality time. We ALL could cause trouble in the air if we wanted to, couldn't we? This guy came out with a smart-ass comment and jeered. What if somebody having just walked through the door of the jet was asked how they were by cabin staff and responded "I'm really p!ssed off at the 13 hour delay!". Whoah........trouble maker alert. Turf him off.
Or, if another passenger before boarding was heard to be having a heated argument on a mobile (with maxalt maybe
). *Whoop-whoop* this guy has violent tendencies. Nope, not getting on.