Why don't airlines install Faraday cages in their planes?
Cell phones and other electronic devices can interfere with the pilot's comms and other crucial avionics


As far as I can tell, nobody ever demonstrated an actual case of this. At least as long as the electronic device is FCC compliant, but all the consumer electronics has to be.

Keep in mind that all the avionic systems already have to be shielded against interference originating outside the aircraft and this shielding works for that in the cabin too.


  Due to these issues, the FCC forbids using cell phones on planes


No. It is due to the issues with the network. Moving mobile clients introduce frequency shifts due to Doppler effect and timing shifts. The system is only designed to handle them for the speeds of cars, but not much higher, so mobile client in an aircraft will not only have unreliable connection, but will also cause problems for the other clients on the ground.

In part, it is also so they don't have to explain everybody over and over why the batteries drain so quickly.


  FAA allows airlines to choose when they may be used, once the plane is out of range of the ground cell towers.


The aircraft is never out of range of ground cell towers, since the range is 35 km and the cruise altitude is only 10–12 km.

FAA allows airlines to choose when they may be used as long as appropriate relay is provided so they actually do work.


  without enforcing it, it doesn't do much.


As long as only effect ever actually observed is that that phone's battery quickly drains and the phone turns off anyway, nobody cares anyway.