The key selected seat memory has to be programmed in using BMW DIS/GT-1...or some of the aftermarket software/harware can do it (INPA and the other software you see mentioned on the forums all the time).
Ditto...this information is NOT controlled by the radio...but as Gregory mentions...the GM (general module) is the main control module that seat memory passes through:
As you can see in this small section taken from the car/key memory settings...BMW designed the system (since 1999 for
3/5/7/X/Z models with 123 seat memory) to be able to activate seat memory from either unlocking using the remote key buttons or when the door is opened. It appears that your local BMW dealership are just not familiar with BMW DIS/GT-1 as far as memory settings for the key: