It makes sense that the M3 defaults to off.
The start/stop feature is linked to the break pedal and in an automatic car you keep the break pedal depressed until you want to move forward.
In the DCT M once I've stopped I let go the break since its engaged neutral like a manual car so it would automatically restart the engine which is dumb and hence defaults to off.
2012 E92-M3 Space Grey on Fox Red, DCT
2011 B8 S4 S-Tronic , Black on magma red