ORIGINAL: landeck
It is to protect them from vibration. If the crystal breaks in the rx or a contact breaks in the battery, you are going to lose the plane. Not necessary so with a boat or car. Note that the new 2.4Ghz receivers to not have crystals and therefore the need to wrap them in foam is not really necessary.
Br;uce
Why is that, Bruce? If it's 72 MHz, you still have a crystal. And you still have vibaration caused by the glow powered engine as well as vibration from driving on land or of boating over the water.
CGr.