Why would it need to be Free? Surely a license for modifying the code has exactly nothing to do with being able to audit it. They could distribute the source in a different manner if you really really really need to be able to see it in order to audit it (professionals don't).
Again, there is no proof OpenWhisper wasn't tampered with. It'd take WhatsApp becoming Free Software and having independent audits to be reliable as a communications platform. Suspiscion is but a matter of survival, and chances will continue to be usually against the most vulnerable party: the users.