Earlier quoted context omitted.
iMessage is extremely secure and utilizes end-to-end encryption, why is this concerning to you?
And it has spam problems: https://www.wired.com/2014/08/apples-imessage-is-being-taken... The problem is authenticity and authority, not encryption. How can the user know this message really came from Apple and not a spammer?
Read the document of the original top post (the document from Apple).
The answer to your question is right there in the document.