score:7
The question of who is to blame is not obvious to answer, as it depends on what actually went wrong.
Looking at the concern you specifically voice, that is not likely to happen. Or rather, pretty much at most once per accommodation as after a host really messes up once, it's not likely the property will ever get another booking (assuming the person booking notifies airbnb of the host's practices). This is also why the peer review system is so extremely useful. So that, if you make a point of only booking peer-reviewed properties on airbnb, you're pretty much guaranteed the booking is legit.
True, you might get burnt once in a while, but this can also happen with more conventional booking engines. There, too, a property with no reviews could end up being a scam.
As far as legality is concerned, the issue is valid, but not relevant to you as a user. It is not up to you to make sure that the owner of the property is legally complying to whatever the relevant rules are. Again, a parallel with more conventional properties exist. Is it your responsibility to make sure, before you book, whether a hotel, say, confirms to the local fire and safety guidelines?