This is very clearly stated on the Prism announcement:
“Prism is currently in closed invite-only beta, but operating on the live Ethereum network. It should not yet be considered “trustless.” The code is not yet open-source (though it will be prior to full release). Further, there is an escape hatch currently built into the smart contract code, which enables ShapeShift to withdraw and secure the collateral if smart contract bugs are discovered. This escape hatch will be removed (possibly at the option of the user) as we become comfortable with the security of Prism in the wild.”
I’m not sure where it is marketed as trustless in its current form.