This is what I find so fascinating. Yes, it cannot be intercepted in transit. But nothing is stopping the keyboard on which I type or the text box in which I type from collecting all the things and sending those things where ever they want. In-transit interception, in this context, seems like a fruitless threat model when compared to the keyboard/text box vector.
With respect to facebook in particular, they have had the means of reading the messages on either side for ages, even before it was required by law. iOS and Android both have "leaky sandboxing", where certain apps from the same publisher can read each other's data and memory, and Facebook added WhatsApp to its sandbox almost immediately after acquiring it, allowing the main Facebook and Messenger apps to read all the messages received by WhatsApp.
