Hi, Kathy!
I know you are correct about them not being bound by ADA, as I checked that out a year ago when we were considering taking a cruise. Talked with a number of people who work with the ADA all of the time and they told me that anything cruise lines do to provide access is not because the law requires them to, but because it is good business.
Maybe this is just me, but I am always concerned about how the rights we currently have can be taken away at any time by a majority vote. That if the majority of people in this country want to remove any of the laws about accessibility, they can. Just by voting for politicians who oppose ADA. Since (if I remember my civics class correctly)(smile), the only rights that we have our the ones in the Constitution, I try to keep in mind that accessibility is not guaranteed for any of us.