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.