What you say is quite true but they AB's can in fact book the cabins by saying they will send the documentation and then just not sending it.
The cruise lines do not follow up on it if they never get the paperwork.
I know people like you and I don't want to take the chance of losing the room and do in fact have to send in the details of our medical conditions, which on it's face is unfair when other passengers don't have to disclose such personal information.
I find the cruiselines will insist on medical documentation as soon as they find out you have any disablility even if you have not requested a handicapped room. Which is really intrusive and not required of others.