No, they can't... not without designing changes into their hardware to allow retrieving the keys from the secure enclave. In theory Apple could modify iMessage

No, they can't... not without designing changes into their hardware to allow retrieving the keys from the secure enclave. In theory Apple could modify iMessage to MITM the key distribution server and enable eavesdropping. The only way to protect against that is to provide in-person validation mechanisms so users can directly compare keys. I hope they add such a thing, not that 99.99999% of their users would ever use it. As far as the US legal system goes you'd need positive law to enforce wiretapping requirements. Courts (as a general rule) can't issue orders to force Apple to write new code or modify their silicon design to support something the government wishes it could have. Given the way SCOTUS has been approaching cell phone privacy I'm not sure such a law would pass muster.