It's because most everyone w/ a BMW drives in a straight line ~99% of the time. Since that's the case, what's the point of having summer tires which you can only benefit from ~5 seconds at a time during on-ramp maneuvers. Not to mention, only lasting half as long as an A/S.

If we all had the luxury of driving through the canyons of Malibu everyday, etc. I'm sure that would be a different story.