For those of you that think this is no big deal, I wonder if you would be ok with your runners giving the middle finger to spectators of a race cheering for other athletes. After all, it's not really hurting anyone and I'm sure the kids doing it would think it's hilarious and all in good fun. That is the justification some seem to be providing.
Every team is different and has it's own set of ethics. What is appropriate behavior for members of one team may not be for another team. I think it's reasonable to expect all teams to include in their ethics the idea that all rules of the sport should be followed. In states where electronic devices of any type are prohibited during competition, this kid's actions would clearly be a violation of those ethics and it would be appropriate for there to be consequences for this.
Where there is no such explicit banning of this behavior by the rules of the sport, then the issue comes down to how the action fits, or doesn't fit, with the other ethics of the team. I would expect this athlete was well aware of his team's ethics and how his actions fit, or didn't fit within them. I am sure there are teams where this kind of behavior would be no big deal.
This would be a big deal, however, on teams that I have coached but I think the kid would have known that and probably not done it. I have had many screwballs over the years that have done silly, crazy things but they have a pretty good idea of when it's ok to be silly and when it's not.
In situations where an athlete is purposely acting outside the bounds of the team's normal ethics, there is more going on than simply having "fun". To ignore that only asks for more extreme behavior in the future, behavior that becomes less "fun" as it becomes more extreme.
For this particular kid, it sounds like the behavior, in the context it occurred, was probably within acceptable bounds of his team. I may not like it, but that's irrelevant.