It's not that it's "disallowed", it's the technology isn't designed to do it. Cell towers aren't aimed at the sky for a myriad of reasons (they use sector antennas), and the protocols are designed at much slower velocities than an airplane. An airplane also has line-of-sight to hundreds of cell towers, which would make cell selection very difficult, and the networks would quickly run out of spectrum since one handset signal would reach hundreds of towers rather than the handful they do on the ground.

Theoretically, the data services offered by planes could host a picocell tower. But the data service used by airplanes is much different than cell networks. Sattelite based services use antennas on top of the plane and have to have specialized tracking antennas. ATG based systems use a network of land based stations and special antennas pointed above the horizon. ATG uses a modified EVDO protocol (yep, that old cell protocol) to work at higher base station velocities.