The OP made the argument that Zuckerberg wanted to know their passwords, such that if the users reused the same passwords elsewhere, then he would be able to log in there and check out their accounts.
For example he could have seen a profile he was interested in, nabbed their password and looked into their email.
Not that he wouldn't have godmode on their Facebook account, and needed their password to access their account, because of course he could have just accessed those accounts without needing the password.
I have not heard this rumor before, though I wouldn't be completely surprised if it was true.