The Code of Federal Regulations, including the FAA rules (which used to be called FARs) do not specify that you have to turn "completely off", it is up to the airlines themselves to set these rules, and the pilots/crew (typically for Part 91) to ensure that no device which may cause electronic interference does so during operation of the aircraft. Many of the ways airlines 'enforce' these rules are arbitrary and useless.

That being said, there is one area most folks don't realize there is something vital about all that theatre and b.s. the FAs come up with - your attention span. During taxi to takeoff, and landing, are the most critical times for plane operations with the highest potential for incidents (like bumping into another airplane, etc.). The crew is responsible for making sure you are safe, and for giving you instructions in the event of an incident. Thankfully they are so rare none of us common travelers think about it, but some crafty planners have.

The easiest way to get and keep your attention is to reduce distractions like phones, laptops and other items.

Of course there is the other aspect of laptops, that they are heavy, sometimes metal cased, and not tethered to your body. If the plane were to stop suddenly, such as by accidentally bumping into the ditch on the side of a taxiway many laptops would go flying if they were out and on the laps of busy travelers. Forcing you to turn it off would I suppose make it far less likely you to surreptitiously pull it back out when the FA returns to his/her seat to strap in for their own safety...