There are more risks than getting shot down. All sorts of dangerous activities may be going on in restricted areas: missile launches, dogfights between fighter

There are more risks than getting shot down. All sorts of dangerous activities may be going on in restricted areas: missile launches, dogfights between fighter jets, tests of unmanned vehicles, etc. You don't want to be a surprise guest at any of them. "Restricted" is a single, short word taught in pilot school to get the message across clearly. If a sudden emergency requires entering a restricted area, then air traffic controllers are there to coordinate it.