I know the overzealous community support officer made this about "abstracting electricity", but let's not get sidetracked by that, because it's not about the cost
As the article mentions, the socket is for use by cleaners when the train is in the station. While it is on the move it can switch from one substation to another. When this happens the feed could change phase and there could be a surge, which could damage anything that is plugged in.
The bottom line is that there is a valid reason the sockets should not be used.