Apple cannot prove they aren't reading your iMessages.
Sure they could. They could change the protocol and UI so that the users could verify and pin keys, and they could publish the iMessage source[1] to allow people to check it and compile it themselves.
Whether they should be expected to do these things is a different issue, but they could.
[1] not necessarily under a liberal license - for example, PGP was proprietary but allowed people to download its sources
Compile it yourself or get some trusted party to it for you, and sideload it. "But you can't sideload iOS apps!" Well, then we're back to what Apple could do.
Apple cannot prove they aren't reading your iMessages.
You said:
Sure they could. They could change the protocol and UI so that the users could verify and pin keys, and they could publish the iMessage source[1] to allow people to check it and compile it themselves.
My point is that publishing the source is not enough since you cannot verify that is the source running on your iPhone. Even if we concede that Apple could let you sideload your own compiled binary (which is not the case today), you still couldn't prove it since you couldn't know that the iPhone is running your binary and not a backdoored binary in the OS.
None of which is to say that Apple should have to prove any of this. If you cannot trust your phone vendor, then you cannot use any function of that phone, be it purportedly-secure messaging or location services or even the web browser.
Sure they could. They could change the protocol and UI so that the users could verify and pin keys, and they could publish the iMessage source[1] to allow people to check it and compile it themselves.
Whether they should be expected to do these things is a different issue, but they could.
[1] not necessarily under a liberal license - for example, PGP was proprietary but allowed people to download its sources