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 And

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.