Doug Miller wrote:

it appears that the plane's transponders were intentionally turned
off, and ...

It leads *me* to wonder why it's even *possible* to do that. What
legitimate reason could there ever be for turning off the transponder
on a passenger airliner while in flight?

Already been discussed in one of these threads.

When on the ground (in a hanger, at a gate, etc) it adds visual clutter
to ATC screens if all such aircraft are squaking their beacons.

I suggested that the beacons have supplementary power through an
interlock with the landing gear - so that when the gear is up, the
beacons are powered and CAN'T be turned off. Normal circuit breakers or
fuses would provide plenty of safety if the beacons malfunction and need
to have their power cut - with said fuse or circuit breaker for the
supplementary power not accessible to the crew while in flight.