That is a fabulous comparison of apples and oranges. Fabulous.
I put a new set of tires on my German cars about every 20,000km, if that. They get noisy and cupped when half the tread is gone, so I toss them. I have 2 sets of tires, snows and summers. Since I run the bag off my car it goes through tires fast.
You are probably comparing "pickup tires" built for good mpg and BMW OEM tires built for traction which means they just wear out faster.
Did you notice which vehicle (BMW vs pick-me-up) stops faster? Corners faster? There is no free lunch so keep buying tires.
BTW check out http://www.tiretrends.com for excellent pricing on performance tires. I always shop there, they are 25%-30% less than dealers and local tire stores.
