[coldacid]

Technically you don't need to ask first, but you still need to tell Keybase directly that you support their proof integration protocol in order to be supported from their side of the connection. Not so much a "please let me in" but rather "I'm ready to rock".

[rakoo]

Linking has to go both ways, it makes sense for the hub to vouch for it. Otherwise you have the same situation as the CAs without CTs: ie A can vouch for me, but I can't really vouch for this vouching, so another malicious B can vouch for a fake me.