[p_l]

There actually tends to be avionics interference from mobile devices, especially outside 2.4GHz band (which is a "junk" band because of airliners - unlicensed use of 2.4GHz band exists so that airliners could use microwave ovens).

Airplanes actually aren't tested for mobile phone interference unless the plane is going to explicitly allow use of in-flight wireless internet. Yes, with modern devices, you have much less risk of interference, but interference happened in the past, and the rule stays mostly to lower the amount of loose, hard and heavy devices that become possibly dangerous projectiles in case of Rejected Take-Off or a crash.

After all, it's easier to put a "theatre" on than try to use reason with barely mentally present (on average) and possibly unruly passengers.