I do not know if BMW shuts off the alternator, they may. My GMC truck does this for fuel economy and reduced load on the engine. There is actually a current loop on the accessory side of the battery cable that monitors current draw and will actually shut the alternator off.

At times you can see the Charging/Battery Voltage drop to like 12.4 Volts, which is NORMAL the way the system is set up on my GMC.

GM had to send out all sorts of Technical Service Bulletins because dealer technicians were replacing alternators left and right for no reason because they did not pay attention to the training info when these models came out.

I also know some performance cars also have full throttle alternator cut outs as well to reduce the load on the engine and try to get more power to the road.

You have to be careful on some of these newer cars and trucks before you write off and alternator and even then many shops have a hard time testing the alternators if they do not have internal Voltage regulators.