The reason is that a bcc'd message doesn't stop people from replying to earlier messages in the thread, whether or not they are using a threading email client. Threaded email reading interfaces probably make your strategy more effective, but they don't eliminate the possibility that people will respond to earlier messages, defeating your "firebreak" message (the more addressees, the more likely it is).

That's true, but it definitely works a charm in Outlook shops — Outlook tries to get you to reply to the latest message in a number of ways.