Maybe the rule being enforced is that all carry on luggage must be stowed. Some FAs regard laptops differently, because a lot of people (not you, i'm sure) take them out, set them up, have their corded mouse out with the USB light and all the other gadgets. Putting them away then takes shutting down, putting it into some sort of bag/briefcase and then restowing said bag.

I have no idea if this is the case or not, but it seems to reason that in the interest of making sure the plane isn't sitting there while someone gets all their stuff settled after finishing a movie at the gate that some FAs would treat all laptops the same and request that they not be used at the gate.

I've never been asked to shut off my ipad at the gate, but since I can stuff it in the seatback pocket (which you can't do with a laptop without starting a fire ) I can put it away in a second or two.