I have interviewed AB passengers who had their travel agent lie just so they could have an accessible cabin, because (the perception was) it provided more room. These folks told me that their travel agent didn't want to lie but "she knew what side her bread is buttered on" and they said they would take their businesss elsewhere if she didn't.
So I find the travel agent just as much at fault. And that is not an isolated incident.
Still, most crusie lines now at least make you self-declare that you have a disability (and many ask what) in order to book an accessible cabin. I think that's as far as I'd like to see it go. I'd hate to see it become a rule for people who want to cruise to have to get a doctors statement (some could just get their doctor to lie for them anyway, so it really doesn't accomplish much.)
I think there should be some oversight, but I don't think PWDs should be made to jump through more hoops (and pay for a doctors visit) just to go on vacation.
Editor, Emerging Horizons
***edited to remove commercial reference*** -- the only magazine about accessible travel