When you pay money to stay in a place with advertised amenities (I'm assuming the swing was part of the listing, idk) it is expected that you can use those amenities without dying... Basically whoever makes the listing should ensure that the product they are offering the user (the abode + amenities) is hazard free.
This article says the tree was dead. The original says the tree was dead for two years. It is no secret that dead trees are hazardous. If you look at a tree which has been dead for years, it looks like it has been dead for years.
IANAL, so I won't discuss the law, but if I were operating that BNB, I would feel personally liable. It was a failure on their part to allow that tree to remain standing (let alone with a _swing on it_) and an ethical failure to take responsibility for other people's safety without fully understanding what was involved.
That said, it is a human error I've made myself (fortunately without fatal consequence).
I am reminded of the parable of the young reformer and the old guard politician. The reformer wants to remove a fence which is blocking a path, because it is inefficient to detour around it and he sees no use for it. The old guard says that, regardless of inefficiency, he won't allow the reformer to destroy something for which he sees no use, because it is very unlikely that their predecessors invested time, energy, and budget to just to frustrate his movements. Taking the fence down would invite unknown consequences on to his constituents.
AirBNB, Uber, and similar start ups are taking a crow bar to regulations they do not understand.
Which is exactly why I see this as an accident. It is very easy to see a tree as something strong and capable of supporting some amount of weight because it has done so for a very long time. Even experienced woodsmen are caught unawares by trees with regularity.
A dead tree is an accident waiting to happen and any tree without leaves is one that you should not climb into or hang weight from or even walk under until you've figured out what the exact state of affairs is.
By their nature accidents are unanticipated. As a general rule we do not proceed knowingly into certain doom. The proprietors of that BNB should surely be forgiven for making a terrible mistake, with no malice in their hearts. No one is born knowing how to run a hotel both safely and profitably (or even that they should be considering safety).
But that is precisely why we've placed barriers to entry for jobs that serve the public, to catch mistakes known to us collectively but perhaps not individually. You're right that it is your own responsibility to verify that a given course of action is safe. But it is also your responsibility not to market defective products, and to educate your users about risks associated with your product. The latter can and should be enforced by society.
Unless there's signs of rot or fungus. And if you're charging people for access to your property and using the swing to advertise you might want to check the branch will bear the weight of adults.
It would be trivial for the owner to test the limb at regular intervals (have a few people pull on the rope from an angle).
The insurance company settled, which suggests that under US law there was liability. Negligence is one of the most common sort of lawsuits in the US, looking it up I see that it is a common law concept and it's hard to find any resources that discuss it in terms of civil law. Perhaps that explains some of it.
