May be this is a design flaw by BMW? Since it did happened with the E46's and the subframe... I'm pretty sure someone along the line did not calculate that after a rain storm on a fair day, may still have water along the A-Pillar there and it may come splashing up. BMW probably believed that you'd have your window up after the rain storm.
Never the less.... I don't there's much you can do but to talk to BMW.
P.S. Still looking for those LED upgrades for you!