Originally Posted by Speed of Sound
Electrical equipment is usually switched off by removing the power to the unit.
Electrical, yes. Electronic, no. Any device which has a data processing capability (and let's not dwell on the depth of that capability) is usually not powered down by a simple switch breaking the power circuit. Usually the off switch does nothing more than sends a signal to the processor, which then proceeds with the shut-down routine. I need not explain more to anyone who is watching this on a computer.
I assume that the ACARS transmitting unit has a built in processor (probably multiple processors), which would go through a shut-down routine (and as part of it, possibly 'say good-bye' to the receiving end). A power disconnect would have a different signature (or rather lack of it) which is why I presume authorities can say with confidence that it was intentionally shut down, not failed.