Ihave often wondered this myself. There seems to be this 2.4G block of channels and Idont see it designated for just air or just land or even just RC. Some people have used "hack modules" to convert their 72 or 75m radio to 2.4 and they dont sell an air or ground version. Although Iunderstand if you get a 2.4 ground radio it wont work with air receivers.
Weird subject if you ask me.