I don't have a problem with this ruling - I think it's a sensible way of making sure inappropriate plugs don't get used. I'd like to see more air-plugs soon though and there is obviously a set of plugs that could be used (like hacksaw or Lt. Macleod). To be honest most of the ground-plugs have effects which either attack ground units; which doesn't make sense in an aeroplane or add ground focused abilities.
Maybe version two will have generic plugs and ground-plugs.
@comediamasta - just a small thing, you may have noticed: none of the plug hero's reward VPs!