I think they did it because when it's cold the shifter is really stiff and it's tough on the hydraulics. With increased revs (3000 rpm shifts), the transmission fluid heats up a bit faster and so will shift better. In really cold weather, if you choose D1 which starts in 2nd gear...the transmission will still start in 1st until it warms up, and it will still shift about 3000 rpms in D1 try it sometime if you have temperatures below 30 degrees).
BMW designed it so the hydraulics didn't break something when it forced a shift when the engine and transmission is cold. Recall regular stickshifts when cold shifting back into first gear...you really have to use some pressure to make the shift from 2nd to 1st.