Essentially Bob acts as the filter between Alice and his other friends. Bob's friends have whitelisted Bob already, and Bob can rebroadcast Alice's public marked messages to them if he so chooses to. If Alice is posting some dumb shit or spam, not only will Bob remove her from his whitelist but he will never rebroadcast her messages. And if he does rebroadcast her spam, the friends he rebroadcasts it to are going to remove him from their Whitelists. And it can keep going outward as well, because Bob's friends can then introduce Alice to their other friends after learning about her via Bob. So it is like a massive group managed whitelist, you can only GET messages from people who you whitelist, but they can send you messages from people you have not whitelisted if they choose to do so and the messages are marked public.