I like the idea of App.net's 'broadcasts'... but why not let the subscriber/follower promote any particular stream to 'broadcast' level? (The implicit contract

I like the idea of App.net's 'broadcasts'... but why not let the subscriber/follower promote any particular stream to 'broadcast' level? (The implicit contract – "this stream won't be so noisy you'll regret its interruptions" – could be communicated and negotiated via other mechanisms. Or via a hinting field – "this stream best for broadcast" – that individual followers could choose to respect or ignore.)