Cell service from balloons legal in the cockpit?

soon to be student wrote:
Why is the use of cell phones illegal while flying in a aircraft anyway??

Two basic reasons.

One is that at altitude, you have line of sight to many more cells
compared to standing on the surface. This means many cells pick up
your transmission and try to handle your call.

The second is that as you move at speeds anywhere from 2 to 8 times
what a car in the highway might be doing, your call gets handed off
from cell to cell very quickly.

Both of these things violate the design assumptions used when the cell
system was built out, and thus degrade performance of the cell network
as a whole. It is for this reason that the FCC (not the FAA) makes it
illegal to use a cell phone in an aircraft in flight.