Actually the "excuse" about terrestrial network interference is the real reason for not allowing cellphones on planes. For years the official stance was that it may cause interference with onboard avionics, but that's really nonsense.
The simple truth is that cellular networks are designed for clients (i.e. cellphones) that are relatively stationary - sitting, walking, or at most driving a car or riding a train. These estimates were taken into account in designing the software that allows a cellphone to move from one cell to another without disruption.
If cellphone usage is allowed on planes, you suddenly have potentially a lot (up to a hundred or more) cellphones moving very fast together from cell to cell. This is bound to wreak havoc with the cellular network.
The solution to this problem, and simultaneously to the big business problem (...) is to install a micro-cell on board the plane, so that all cellphones on board will be using this (relatively to them) stationary cell, which will communicate with the terrestrial network via a specialized link (like the one servicing the on-board phones today). The open problem then is how to make sure all phones use this cell and not the terrestrial ones, as some frugal smart-alecs are sure to direct their phones to use their "home" network and not the "roaming" one on the plane.