Hamster:
You are forgetting that transponders and ACARS were switched off before the last communication with ATC.
It is 'known' (or, we have been told by the Malaysian authorities) that the transponder and ACARS
stopped transmitting at different times
prior to the final communication with ATC. It is, however likely, still an assumption that they were actually switched off. What actually occurred to stop them transmitting and the reasons for this occurring, are still the subject of conjecture.