I believe it was because before the recording started (and you can hear the woman who is recording quietly talk about it) the kid was using very loud offensive language, as in curse words, and I'm sure he was being threatening as well. Usually offensive language is swearing and is frowned upon in an area frequented by young children, especially. It usually isn't enforced, but in this case it was. The arrest wasn't illegal since it was clearly required to get two belligerent customers out for other customers' and eomployees' safety.