Thanks for the links! I enjoyed reading about how iMessage is built on top of APN. That probably explains why I can reply to messages in arbitrary apps on my Apple Watch. :-)
However, that doesn’t change my argument. Beeper is not a trusted party in this exchange. When they show my messages to their users, they are decrypting my messages and user activity in a way that is outside my zone of trust. They can then be nice and show it to their users in their app, or they can be nefarious and send that data to any other 3rd party for whatever purposes they want.
This is a major security hole at the application layer, despite the network layer security that you’ve linked to.
This strategy is only okay until you reach act 3, and then it gets crazy powerful with all the items that bump your spell save DC up.