I'm responsible for making sure 10 or so websites are professional and are ready to represent the brands of a $100+ million dollar company. It's a big responsibility. I once said I couldn't believe those idiots would trust me with that task. Does that mean I think my employer and my bosses are idiots? No. It's just a way of expressing humility and coping with the fact that a task comes with a high amount of trust and responsibility.
In my case and in Zuckerberg's the comment was made in private and never meant to be heard by the public. I think using this quote against him is a cheap shot and a total copout to avoid answering the parent's question in any real way. If everyone's private comments were held against them then every person on earth would be vilified. I'm no fan of Facebook but I am a realist and when you try to use that quote against him you come off as blindly anti-Zuck/Facebook.
So to rephrase the question, why do you think Facebook users are fools? And if the answer is "because mark said so" then I'm sure I can find Zuckerberg being extra complimentary of Facebook users and then I'd ask "so which quote do we go with? The one that suits your belief or would you prefer to come up with your own reason?"
You should perhaps do some further reading into the source material. The op asked why this was a prevalent view, I supplied an independent analyst citation, and quoted some relevant material. That is reportage, not my personal view per-se, and it has been the former that has been more influential to date rather than the latter.
Clearly, this is a view that some people hold, and that is evidenced based. That evidence is quite damning when considered <in context> both at the time--he's offering to distribute private, personally identifiable information without consent--and in the larger, later context of FB's evolution, and subsequent business decisions. It is not a "cheap shot and total copout". On the contrary, it is dead on point. As such, it seems a leigitimate position. Playing devils advocate, is also one that at least needs to be argued against rather than wished away.
If you have evidence or logic to argue against it, and that is a position you believe, then feel free to put it forward. On the other hand, putting forth your own foible is not overly persuasive. That seems, to the contrary, to have been a mere expression of incredulity.
 The analyst is short facebook, obviously. (And per her disclosure at the end of the linked citation).