It’s a weird feeling to see Facebook fighting on behalf of users privacy, but I suppose they want to keep their monopoly on their user’s data.
It’s that, and it’s also two more things I think: one, there’s public demand for encrypted apps, so it’s part of how Whatsapp markets itself. And two, Facebook really wants to be perceived as caring about privacy, especially after the Cambridge Analytica scandal. They actually hired away a few people from Access Now and EFF to go work for them. I am deeply skeptical about what those people can actually do in those roles, but I know that Facebook loves to point to those people as proof that Facebook cares!