Of course they shouldn't have to have a wheelchair levy, but it's an elegant way of screwing some more money out of the punters.
According to the Ryanair news release archive (see 6 Feb 2004 under "News" on
www.ryanair.com), they are charging £0.35 or €0.50 per passenger sector as a wheelchair levy, presumably now to be halved. They also say that the cost of providing wheelchair assistance at STN (which they have to pay) is £18.00 (which I assume will now also be halved).
So that levy covers 1 in 36 of all FR passengers needing wheelchair assistance at an airport that doesn't provide it to FR for free. And I'm sure that's a perfectly reasonable ratio - I mean, if on average there were fewer than five wheelchair passengers on every full 737-800, that would mean Ryanair was profiteering, and that could never be... could it?