Simple.
It is illegal to use a mobile phone from an aircraft. (excepting licenced systems installed in airliners, obviously). To do so is an offence - probably a criminal offence and for very good reasons against the Wireless Telegrapy act of 19**. (related to a historical rule about antennae attached to elevated structures I believe)
You may choose to ridicule this but it doesn't alter that fact that you may not do so, and of course responsible citizens won't.