As you've pointed out, there are plenty of places where it is perfectly legal to do this. My guess is that it's harder to find a profit opportunity, though, because the more profitable use (legal use as a short term hotel) is already priced into the unit.
I suspect that one reason airbnb is so profitable in areas where it is not legal is that the restrictions on short term rentals suppress the sale price of the unit. So you buy a unit that is cheaper precisely because it is illegal to use it as a hotel, and then you profit by using it as a hotel. So in cities with very strict zoning and tenant protection laws (like SF), I guess they're selling the assumption of risk.
