In my oh-so-humble opinion, so many people here are missing the point.
Ultimately I think that this is about social responsibility. We all have a responsibility, I believe, to get along and reduce the sh!t that we ALL - society - has to deal with.
I think that all this talk of assault, leave it to the BTP etc, is wide of the mark - it's passing on the responsibility of dealing with someone who (for the sake of argument) is acting outside the boundaries of society believes to be right. It's our responsibility - mine, yours - to point out when things are f*cked up and wrong.
That situation was f*cked up and wrong. The choice is to deal with it yourself, or pass on the responsibility to an "authority" in the matter - wait for BTP to eventually come along, inconvenience the whole train for the sake of the sensibilities of some young scrote.
If the young scrote in question believed that he couldn't get away with sealing the ticket and that he wasn't untouchable, that situation might not have happened.
* I'm making the assumption that he WAS a proper fare dodger, and WAS a young scrote, based on the fact that he looked like one and I'm never wrong. If I'm wrong in this instance, I apologise.