Acropolis, you posted a picture of the specification for a hinged doorway, not a doorway. They are different in that the hinged doorway has a door. You really need to spend some time learning the software AND the terms used. A doorway has no door, it's just an opening between two rooms or even interior to exterior. In fact, if you would turn off the interior casing display, grab the remaining door in the specification and spin it you could see both sides. The casing will be GONE on both sides BECAUSE it's an interior door. Edit: By default, the interior side of the door will face the direction that the door swings to open. If you prefer, you can specify that the interior side face the opposite direction in the Door Specification dialog. However, in a doorway, again there is no door. In that instance, the 3d preview in the specification dialog concerning interior vs exterior eludes me still. But it's really not important if the idea is to remove the casing from both sides.
If you can't accept that a greyed out choice only means it's not relevant in the current context, then I think you are out of luck. There is no fix as none is needed.
Post a plan. Perhaps there is a problem, and semantics is hard. You CAN deselect exterior casing on an exterior door but only on an exterior door.
I just saw DavidJPotter's video in which he goes over these very items in a very easy to understand manner in response to another of your questions.
Doorways.plan