4-stroke engines need to maintain atmospheric air pressure in the bottom end of their engine, particularly on a single cylinder engine they literally breath in and out every time the piston goes up and down. If they can't breath in this manor, such as if the crankcase vent becomes completely plugged, they can become an air pump and that will be a bad thing. It is important that the air going in is clean and free or water and when it breathes out it expels a very tiny amount of oil mist and possibly water from condensation that might otherwise accumulate inside the vented crankcase. One of the reasons for gasses to blow out and be vented into the air box is the result of piston blow-by (leakage of combustion gasses past your piston rings) This is totally normal even in a new motor and not a source of concern provided the amount of oil is not excessive. Because the drain pipe is directly connected to the air box after the air filter, you wouldn't want dirt to go in there, hence the one way drain valve thingy.