The problem with your definition of a convention is that it's impossible to distinguish from an idea that you - and you alone - have.
Well it's a good job it's not an idea Bletchleyite alone has. You also have it, because you've seen it yourself.
I've seen people using the sockets to charge their phone that don't have a sign on them in pubs, cafes, libraries, motorway services, airports and yea railway stationa on a frequent basis.
It's
conventional that you can charge your phone in this way. Convention frequently ignores the law. For years it was conventional to steal films and music online. On some tram services it's conventional to travel without paying for your journey. It doesn't make these things right, or un-punishable.
I would suggest there is no harm in people charging their phones for free, though. Totally unlike failing to pay for your tram journey.
A great many stations have sockets which
are for use by rail passengers, including dedicated ones installed for the purpose at desks, and signs stuck on ones that have been there for years down at floor level.