That's what these large corporations all do.
Look at Google, grandstanding about moving things to HTTPS a few months ago, making things harder for the NSA, and so on, and yet at the same time they are now proactively scanning people's data for illegal activity and then handing it over to the government. Microsoft is doing the same thing.
What makes you think Yahoo will do anything different? The whole plan here is probably to get uninformed users to hand over their PGP keys so they can store them.