They could. Correct me if I'm wrong but users don't see which public keys have been used to encrypt the message's symmetric key. Theoretically Apple could easily and invisibly include themselves as a recipient.
You're exactly right and I'm not sure why this is being downvoted. Apple can add additional keys to iMessage messages and thus view them in transit - they say this themselves in their own security white paper[0].
criddell|8 years ago
nstj|8 years ago
[0]: https://www.apple.com/business/docs/iOS_Security_Guide.pdf