It's not the transmission causing this, but the torque converter.

http://auto.howstuffworks.com/torque-converter.htm

A torque converter needs a certain amount of RPM's before enough torque is transfered to cause the vehicle to move. At idle it is possible for the car to roll backwards downhill with the transmission in drive. Be careful though, if enough speed is built up you can stall the engine and then you have no power assist to the brakes or steering.