They do get given leeway. When I was a guard I had a 3-in-1 cable and powerbank for exactly that purpose. Discretion is used plenty of times.It's rather bad luck if you board a train after being out all day and maybe taking photos etc., to find the power sockets aren't working. I think people in this situation, who would normally have been able to recharge their phone on board, but due to the train's fault can't do so, should be given some leeway if they genuinely can't display their ticket. I'm sure some websites (can't think which at the moment) show such information as whether there's wifi and charging sockets, so if you board with an expectation that isn't met then it's hardly your fault.
You just don't read threads about it on here.