Unnecessary network traffic
When the phone is moving away from the area covered by one cell and entering the area covered by another cell the call is transferred to the second cell in order to avoid call termination when the phone gets outside the range of the first cell (http://en.wikipedia.org/wiki/Handoff).
Imagine an airplane in a waiting loop over London. If 300 passengers had their phones switched on, the plane traveled at 400km/h and the range of a mobile cell was about 10km, this would result in 200 hand-offs per minute (400/10*300/60). Multiply this by the number of planes in the air at any one time and one can get an idea of the magnitude of un-necessary cell traffic which mobile operators would have to handle without being able to charge the user for it.
Each mobile phone cell has an upper limit on the number of calls and the amount of traffic it can handle. Once this limit is reached, additional traffic cannot be handled and results in dropped calls.
Hundreds of potential hand-offs can generate enough additional traffic volume to temporarily drive mobile phone cells to or above their maximum capacity, resulting in dropped calls or user's inability to initiate/receive a call.
The problem could be solved by installing additional cells and by upgrading existing ones but since mobile phone operators do not charge their users for hand-off traffic, there would be no financial benefit to these operators. And if it doesn't generate revenue, it makes no financial sense and consequently it won't be done.
Thus, I don't think that allowing airplane passengers to use their phones from airplanes is something which mobile phone operators are likely to endorse or encourage.