I'm have a vague recollection that there has to be a minimum charge of some kind (1p) for to make a contract. As they do not sell return tickets (just 2 one way flights on one booking) hence it becomes 2p if you book a return.

Therefore it's "free" or almost free as much as is possible. I'm sure someone will correct that if I'm wrong.

And yes - this is why they allow Electron to be fee free (which I wouldn't say is "obscure", just not always issued in the demographic of the average Flyertalk visitor).