While I agree that these should only be released to those with a need. I believe the doctors letter should be sufficient. It is already putting an obligation to disclose private information that other cruisers do not have to provide without making it even more onerous on those of us who do need them.
The lines do try to keep the cabins and they are suppose to only be released by special services until shortly before the cruise when they are released. but there are two problems.
One is as you say unscrupulous TA or travelers who say they need it and will send the documentation, then they don't and once it has been released the cruise lines rarely follow up if paperwork does not show up in a timely fashion.
the second problem is with the booking software in that say you need such a cabin and you book one, then for some reason you cancel the software then puts that cabin back into regular stock instead of returning it to special needs dept. this is a big problem.
If the ADA ever gets the laws in place requireing the cruise ships that stop in US ports to follow certain rules I think we will see this get better. but until then all we can do is hound them.