How exactly this is an example that proves necessity of government regulating private rentals?
Where in the social contract is this clause? I can't find it.
"It's in the social contract" is the ultimate weasel appeal. You're citing the authority of an imaginary document that, miraculously, contains whatever arbitrary moral edicts the person who invokes it happens to agree with.
Transactions should be voluntary. People should not be forced to do business against their will. Whoever "we" is in your post, I'm not part of it, because I have a moral objection to slavery (I.e. Involuntary labor).
Not that what AirBNB is doing constitutes involuntary labor; they're a private business and can set whatever terms they want for using their platform, in accordance with the principles I've outlined. I'm just saying that forcing homeowners to take in people they don't want to is sub-optimal and alienating for homeowners.
Airbnb has found (accidentally) an unforeseen loophole in the law, where a practice that had only a marginal impact on the distribution of housing is on track to make a more significant impact. They know as well as anyone else that the law is going to catch up, unless they keep human nature from using Airbnb as a vector to re-segregate a big chunk of the housing market.
We tried it the other way, and all the black people ended up cordoned into North Lawndale.
http://www.theatlantic.com/magazine/archive/2014/06/the-case...
As a society, we decided to amend the social contract to remedy this. You can choose who to have in your house based on any criteria you like. You cannot choose who to rent to on the same terms.