It does also works as airbag sensor, then the passenger airbag wont go off, if there isn't any passengers in the seat.The switch is for baby seats indeed (the passenger seat does have sensors, but for seat belt warning). When the switch is set to "off", an indicator light is lit where the rear view mirror is.
As for the gap, is it not strange that at the dealer's all the cars had no such gap? Maybe they have carefully chosen them for that reason?
On a personal note, I must say it is rather disappointing to find such a QC issue/design flaw on a (presumably) BMW..
Its not a design fault, probably just a bad door seal. Get the dealer to replace it with a new one, then you will be good to go.
It's not a known fault, you are probably just unlucky. But it's a "light" fault.
