There are payments specified in the protocol, although I haven't implemented that yet. You can see it when you create a Superball. Yes, a Thrower could censor you, it which case you would simply post again and not use that Thrower. I have included an audit tag. The audit tag allows you to follow the bounces in the relays to see if the Throwers are doing what they should. Following the audit tag decreases your privacy for sending actual notes, and should probably just be followed up on for verifying that Throwers are not malicious.