Are you suggesting that passengers are required to be alert, undistracted, and focussed on safety procedures as they taxy out and take-off? Where does that rule come from? Let them relax, that's what they want to do.
If it's the use of the PDA that bothers you, there are plenty of electronic devices in and around the cabin, my watch for example, that perhaps should be turned off, but that's just not practical.
The 'offender' is a customer - to be treated as such - not told off by a petulant control freak!