Don't see why not listing the public address would make you paranoid that they didn't even hold the BTC or something. They aren't concerned about you getting the private key, but maybe they don't want to disclose, when they don't have to, where it's located, how it's distributed (one on address, or many), how much others have invested, and the paper trail of where that BTC came from and moved. Seems reasonable to me really..
Reasonable guesses; it will be interesting to get SecondMarket's response.
Would there be a way for someone to "sign" a message proving they have control of a certain amount of Bitcoins *without* revealing any public addresses? Is this akin to what I've heard discussed called "Proof of Stake"?