Book asumes that every country is played by a different player!!
So if you Own a fighter in a zone that is bombed you can chose to scramble it.
You cannot force other players to scramble but ofcourse you can ask them. If you play multiple countries you get to deside for each country if you want to scramble and how many.
That fighter can be used later in your turn again to fight ( if it survives ).
Think of it like being attacked in a country, those units defend now but you can still use them in your turn.