The whole point of Ryanair - and other European low costs - offering "free" booking using Visa Electron is that it gives them a legal way of having low headline prices. However compared to other forms of payment few people have an Electron card so Ryanair and the rest get a "nice little earner" whilst still being legal.

Of course Ryanair is the only one that charges the fee per passenger per flight so a couple doing a return trip will pay four fees