iMessage is Apple's service, and they can do with it whatever they want. No other arguments are really relevant.
As for whatever reasons Apple comes up with: that is probably also not going to be relevant as a multinational that is beholden to money is going to have the legal department and PR do that sort of messaging and not anyone on the technical side of things.
Speculating as to why things are the way they are: Apple knows that people in some socioeconomic ecosystems value iMessage as-is, so we can expect their intent to be aligned with keeping that value. Reusing all in-house crypto and account management certainly makes it easier on the engineering side as well.
And that's where that 'if' is important: iMessage isn't very relevant outside of the US. Worldwide it doesn't even reach the top 5. Inside the US, even Facebook Messenger is apparently used more than iMessage.
As for whatever reasons Apple comes up with: that is probably also not going to be relevant as a multinational that is beholden to money is going to have the legal department and PR do that sort of messaging and not anyone on the technical side of things.
Speculating as to why things are the way they are: Apple knows that people in some socioeconomic ecosystems value iMessage as-is, so we can expect their intent to be aligned with keeping that value. Reusing all in-house crypto and account management certainly makes it easier on the engineering side as well.