The risk has always been there. Have you ever seen/used a lock-pick gun? They'll open anything non-specialized in less than a second. Failing that, a lightly skilled (50h practice) lock-picker can open most household locks, even "secure" ones, in under fifteen minutes.
Beyond that, the lock/deadbolt/wall is pretty weak. A sledge hammer will open almost any door in a single blow. Windows are essentially just big gaping "serial-killer's enter here" signs, as you can easily break them and gain access to the domicile. Where, presumably you would feed on the brains and organs of the residents.
And worse, the phone company publishes a book that lists your name and address. No longer do serial killers need to drive endlessly, trying to stumble upon a Stuart or a Fred, they can just pick up a phone book and they'll find a perfect list of people, indexed by name for easier killing.
PANIC!
But seriously now, you're over-reacting here. These risks are everywhere and AirBnB was just another way to meet another person.
What I'll consider unethical is if they do anything differently for this lady, who got the media attention first, than they'd do for anyone else.
I don't necessarily think they're liable, even morally. It depends on how they represent themselves. If they purport to offer screened participants and don't, that's a problem. But if they don't it simply means they're offering a lesser service, pay accordingly.
I want a market to exist for cheap and potentially dangerous things instead of offering only padded kid-safe versions. For instance, Lithium-polymer remote-controlled vehicle batteries. Essentially the same tech as the laptop/cellphone batteries that occasionally catch fire. But they don't have protective circuitry built in to prevent this. If you overcharge them they will catch fire. But they're far cheaper and you can draw more power than if they had safeties built in. If you're clear about the trade-offs what would cause a recall and lawsuit in one market can be perfectly acceptable in another.