To answer the OP more precisely, not every tank that is in the game could operate the turret ring without the engine running.
Also, the German tanks had the ability to start and stop their engines at will, except after a prolonged period of sitting with engine off. The American M4s would have to run at idle for about two minutes before moving, and if they shut down too quickly (without likewise idling for two minutes) would foul their engines or overheat.
The M4s also have an internal generator, so they did not have to have the engine running to operate the turret.