So, in all honesty, YOU can't really say how dangerous operation of an FM
receiver will be; but you KNOW that it's potentially harmful.

Given that the aircraft voice comms are just above the FM BCB, and the
typical first IF is 10.7 MHz, it's not too hard to imagine the LO sitting
right on the tower comm frequency.
You may only radiate a microwatt, but you're much closer to that antenna on
the aircraft than the tower is. Inverse square law makes it very easy for
you to win that contest.

This is a pointless argument though. It's a health and safety issue, and
you either follow the airline's rules, or I hope they boot you off the plane
(optionally, landing first for your convenience) It is just that simple.