This actually sounds reasonable; most other messaging programs do the same in order to provide previews, thumbnail images, scan for malicious links or spam, etc.
[0]: https://www.apple.com/business/docs/iOS_Security_Guide.pdf
I just read the section on iMessage (from around page 49) and I can’t see where this is written. Can you point to the part where they say this?
> The private keys for both key pairs are saved in the device’s Keychain and the public keys are sent to Apple’s directory service (IDS), where they are associated with the user’s phone number or email address, along with the device’s APNs address.
For Apple as a company, not having access to iMessages is the safest thing to do, and I believe them when they say they can't access them in the current setup and are not willing to change that. It's because this would change their status from hardware/software vendor to telecommunications provider, with all related problems and costs - and they don't need any of these, so the best option is just to shield themselves from any user-to-user communication.
If you send a group message, Apple provides your messaging client with all of the recipients' public keys that are used to encrypt the symmetric key that actually protects the message. They could slip their key into that list and I don't think you would be able to easily tell if they did that.
If you send a message to a single person, then that's just a group of one.
The interesting question to me is if Apple can be compelled to write code to do this if they haven't already done so (and I don't think they have). I wouldn't think they could be forced, but like Microsoft did with Skype, they might do it anyway.