Part of the venting is to let air into the bathroom for the exhaust fan, and part is for HVAC. The bathroom gets hot or cold and very few coaches have hvac vents in the bathroom. A full door will prevent the heated or cooled air from circulating into that bathroom.
In some RV's, the bathroom door also swings open to divide front and rear living spaces. In that case, a shorter than full height door also allows heat and A/C to mix front to back.
I suspect the top bunk view of the bathroom is a combination of a floorplan the door wasn't designed for and being too lazy/cheap/whatever to modify the doors just for that model.