Well, there are a few issues with that.

One is that I've only been asked to turn off such a device when the plane is taking off, landing, or taxiing. I assume that is because interference with avionics or communications equipment is critical at those points. If there was an issue during the flight, there is time to tell passengers to turn their devices off. Why prohibit an item if it can be used at certain times during the flight, since most of the people on the plane are (presumably) responsible adults.

The second is that people may want to use those devices in the terminal, since they have to arrive early and may have long layovers. Why prohibit an item if it can be used at certain times off the airplane, since most of the people are (presumably) responsible adults.

Then again, that's quite the presumption. Apparently we aren't responsible adults if we aren't willing to put away our gadget for half an hour because someone feels that it may pose a safety concern. The rules may seem overbearing, but that is because the costs are high if an airline does go down (both in terms of lives and dollars).