Sure, your own residential roof is a different story, as is a dinghy.
The difference to me is the expectation of protection from obvious hazards in public and commercial spaces.
In addition to monitoring (a last ditch effort imo) it seems relatively trivial to install nets or fencing to prevent falls off cruise ships. Why that isn't alreay mandated given the statistics provided is beyond me.
Going further, with horizontal netting or 6 foot fencing, would be trying to protect passengers against injury even in the case of outright thrill-seeking or tomfoolery. I'm guessing the cruise lines resist that partly out of cost considerations and partly because they don't want the ships to start looking like prisons. Passengers aren't going to like it if they bought a luxurious getaway and they find themselves in a cage.
If someone sober took her out on a dingy and just left her there after she fell overboard, people would be pretty outraged.