This probably has to do with whether or not a device is able to participate in Skype's p2p call routing. If being on 3g introduces too much latency or has too l

This probably has to do with whether or not a device is able to participate in Skype's p2p call routing. If being on 3g introduces too much latency or has too limited bandwidth to contribute in the network, then each client will have a marginal cost to Skype. Maybe they're trying to ensure they can keep up with that. Or maybe they're just grubbing for money.