Bad example, because one goes to HN specifically for curated content.
> Would you be opposed to an airbnb clone that only rented to white supremacists?
Lets run a few variations of this, and try and find a consistent position: Would I be opposed to HN deciding what content is allowed? To a nazi Airbnb clone? To the New York Times deciding what they publish? To every newspaper in Nebraska colluding on which stories they allow? In secret? What if the top 80% of websites, by views, do so? What if Facebook does it? What if your local car dealership doesn't want to sell to communists? What if Toyota doesn't? What if an alliance of car manufacturers representing 98% of global car sales doesn't? What if a taxi service with 5 cabs to it's name refuses to drive Hillary supporters? What if it controls every cab in New York? What if Intel requires you to use their own Linux distro, and bricks your chip otherwise (they warn you before you buy)?
In all these cases, the answer is the same - it's only troublesome once the company, corporation, shadowy behind-the-scenes cabal, whatever, gets too big. Once going to a competitor is no longer viable, or very difficult, or something you don't even know you need to do (sure it's easy to go to death2amerikkka.ru/imperialistPigAggressionInSouthAmerica/latestNews/, but you don't know of the site, and have no reason to distrust the CNN/Fox talking heads parroting the government's position).
So to answer your second question - no (at least not beyond 'they shouldn't have done that, but I guess it's their right'), but only if there are viable alternatives for renters. When not using their service becomes very disadvantageous, I'd be very opposed. Of course being opposed doesn't say much - a better question is, what should be done about it? The answer is known, but neglected - fire up the dusty antitrust laws, and do what was done to Bell and Standard Oil - don't just prevent mergers, but break up the companies.
Sorry for the wall of text - I try to be concise, but failed in this case. Maybe next time ask easier questions.