If I were to guess I would think it has more to do with the bolt action lock than the door handle lock.
I do know that if you "only" lock the entry door handle, there are master keys out there to open these (RV dealers have them). But if you lock the bolt action lock there aren't master keys for these. Of course the same "owners" key is mastered to open the handle and bolt lock on a specific door.
There may also be something to do with they way they are manufactured, and like Lee stated, not all TT's have two entry doors.
Just guessing, maybe someone knows the "real story"