I don't think it's as silly as people are making out. It at least proves a passenger is in control of the device, rather than it being stashed / hidden in the cabin. A device in the cabin not owned by any passenger broadcasting a signal is definitively more suspicious than one with a passenger in control. We don't know what their next step would have been - they might have asked everyone from row X to Y to turn their bluetooth back on to narrow down the search etc. They probably didn't expect that anybody would fail to respond to the first instruction.

Turning it off would have solved the bureaucratic problem for flight crew. Sadly, the passengers (collectively) failed to accomplish this basic task.