Exactly - in which case, I don't understand why AirBNB allows listings in those places, or at least warn you. It's a terrible user experience to find out you're an unwitting co-conspirator in someone else's crime.
Because they don't give a shit. They make money on the rental anyway and as long it doesn't threaten to wind up with terrible PR or cost them tangible money why would they care?
I used AirBnB twice in Japan and booked a couple experiences (which I thought were really expensive for what they were) and I always cringed at their email sig :
From AirBnb with Love
It's not about spreading love. It's about a pretty ruthless business. Externalities be damned.