I think the argument for including stationary drivers is the tenuous link between answering the phone whilst driving, stopping to deal with the call, and causing an accident or obstruction whilst so doing.

Whilst this could happen, booking people simply because they are using a mobile whilst in charge of a motor vehicle is stretching things too far.

The law was introduced for safety reasons, and a stationary vehicle is only causing danger if creating an obstruction - and there are already laws to deal with that.