I'm not going to defend Quora's actions, but your comment sounds like knee-jerk cynicism to me, especially if you don't know the founders personally, and that irritates me. If you've started a start-up before, you know the mix of a big vision, good intentions, and also some desire for fame and/or fortune that varies from founder to founder.
Starting a site like Quora and painstakingly nursing a community of intellectuals into existence would be a pretty dumb way to make fast money, given all the possible business models for doing so.
I would guess that since it is socially uncouth to say their motivation for the site is to try to make a lot of money, they instead state a more noble sounding mission whenever asked about motivations.