I also would find this unacceptable. If short trips are enough to do this, then either BMW (a) under-sized the alternator for the electrical system, or (b) has gotten some alternators that are not putting-out their rated amperage.

But first off, find-out if the battery is weak or undercharged by measuring the battery voltage with a voltmeter.

This has a table for approximating the battery state-of-charge by voltage:

Battery FAQ

But the best time to measure the battery voltage is after it has been sitting all night -- right after you stop, the battery has a false "surface charge" that will give a high reading. If you don't want to wait, then turn-on the head lights with the engine off for a bout 30 seconds, turn them off, and then measure the battery voltage.

Now, you shouldn't have to do this, as it's the dealer''s job, but I posted it just in case you want to get some data yourself to help in your discussions with the dealer. My experience has been that they do a more careful analysis if they believe the customer has some understanding of what's going on with the vehicle. Talk directly with the mechanic/technician who works on it, if possible.