Keybase has created an inbox in your name which in turn creates a social contract on your behalf to check it. Existing users signed up for something different, so no wonder some of them want to disable that feature.
Again, I agree with the feature-creep point. What I was asking about is why is the connotation about private messages seem to imply that they don't think encryption is sufficient for a third party to hold a copy of a message they will never read.
Not really. PFS is about protecting a long-term key from being broken and then historical communications being uncovered. If you receive a one-off message then it's not materially different to being PFS with just a single message.