LOL!! You would think BMW engineers looked into this and everyone wants to add flapper valves, block off an engineered air-intake vent. For real fella's. The water you see comming out is water that has built up arond the kidney hole and is pushed out of the hole due to airflow over the hood and air that is comming through the airbox pushing that water from around the rim out. Not the airbox filling up with water.
It's possible under heavy rain conditions for water to run up the inner-lower portion of the airbox being pushed by airflow through the front vent. But the box is designed so water does not touch the air filter and will not affect the engine.
I'm sure if someone drove into a deep puddle you might get a different result but it's perfectly fine. For as much as it rains in the UK, I've had no issues. In fact, I even opened up the filter box after driving in the rain one day and the filter element was bone dry.