There are some urban legends, Federal (USA) laws and regulations, and a bunch of brown matter that is better flushed going around. That said, I will share what I have learned.
1) after speaking with the FAA Regional Flight Surgeon for the Southeast US, she and I agreed, there is no need to turn any radio frequency diabetic devices devices off during a flight.
2) a discussion with Delta's Flight Surgeon had the same response. Do not turn off the radio chatter between diabetic devices during a flight in the USA or on a US Carrier.
3) I have been told to put up my PDA when checking a BG during "restricted times" on US flights. Once I share what it is, everything is fine. I have even been asked for a "show and tell" by the cabin crew.
4) be careful flying into other countries or on non-US carriers. This is the reason Medronic has two different pump/remote models - one for USA/North America and a second for the rest of the world (Animas and Omni-Pod do not to my understanding). I have posed the question to a local representative about using USA equipment in other countries. Let your imagination go here about secret transmitters, spies and the wild spin a writer could pen.
Facts related to the situation:
1) Any radio transmitter or receiver on an aircraft must be both FAA and FCC "type accepted."
2) FCC for the most part works well with FDA since the devices are low powered and usually on telemetry frequencies (medical or industrial).
3) FAA wants its own, very expensive test performed at the expense of the manufacturer. This has little pay back for the manufacturers.
So now to answer your question "why MM wants us to turn off the CGM during flight ?"
The reason the manufacturer of radio transmitters within medical devices tells users to turn off the radio features is because the manufacturer does not have an FAA blessing for the equipment to be used aboard an aircraft operating under FAA control.