We know about it because its been reported and they said they are going to do it.
Well if it’s been reported and is actually true, I bet it’s within the terms and conditions (if you want them to search your stuff, they have to read your stuff). Let’s revisit when it becomes clearer.
A clarification or whether or not Microsoft controls Win 10 users’ private key (not really) and what they can do with them (nothing) has been posted here:
The Register Biting the Hand that Feeds It!