[Tzfat]

Ok, my dumb question— why are transponders so easily turned off on commercial airlines?

Because they are for use in flight. They are turned off, or standby on the ground. If you are concerned that pilots have too much access to important controls on an aircraft, that train left the station almost 111 years ago.

[nascarnation]

I thought the transponders were turned off by pulling circuit breakers?

Sort of like disabling the anti-lock brakes on your car by pulling a fuse.