I haven't read the article (and don't intend to), but I bet this "law" has been in place for years and a warning/citation is the normal method of dealing. I will also bet there was more to it than just eating a fry. I can see the picture now- Kid gets asked/told to stop doing that, kid gets belligerent (my friends are watching, I can't back down, that wouldn't be cool), escalation happens.

Or the train crew could just have been dicks.