That's probably more to do with the US military owning that part of the spectrum than anything else. There are different channels used in different parts of the globe for 802.11: List of WLAN channels - Wikipedia, the free encyclopedia
Not sure I understand. There is a lot more to compatablity than final transmitted/received frequencies.
Aircraft system components (computers, radios, etc) from different manufacturers should have similar characteristics to suit their intended purpose (e.g. navigation and communication systems should cover the same frequency spectrum), but the regulators are not willing to take the chance that components not on the "tested ok" list behave exactly the same as the (already) tested ok components (i.e. tested in conjuction with this brand/model of wifi system).
Instead of having to prove that the wifi system is harmless to every known aircraft device/component (from every manufacturer), the wifi only has to be proven to be harmless to a limited range of aircraft devices/components (from specific manufacturers).