I don't know the reason for the typically high tire prices at BMW dealerships but I don't care about it becasue I don't buy tires there. Get em by mail and use a local cruiser/sport bike shop or a friend's place. Sometimes the local shop I use can match the mailorder price so tires can be bought there..
I buy BMW parts from my local dealer or one of te larger mailorder places, depending on what I need and where I am. Prices aren't always the cheapest but I can't claim to hve been gouged by anyone. I like the fact that it is easier to get parts for old BMWs than for relatively new J-brand bikes- that's got to be paid for somehow. Besides, I try to learn something from the shop guys when I visit a dealership..