there are sealed batteries, and there are vented... basically, vented batteries are NOT sealed, therefore, the fumes from the internal chemical processes are not contained in the battery itself and are released outside... the battery compartment essentially is located inside the cabin, therefore, the toxic fumes may accumulate in concentration that can be harmful to humans, can settle in the surrounding areas of the plastic tub where the battery is located - think of leaking battery in the old transistor radio...

besides, venting the battery is not a tremendously difficult task - why would you not do it in the first place?