By default, anybody who messages your page is subscribed to the global broadcasting list. Saying that, certain growth tools require the user to respond to the initial message from the bot. These tools include the JSON, CGT and the checkbox plugin and widget.
Keep in mind that when somebody first messages you, they must click the "Get Started" button to confirm messaging.
Since Facebook considers this an explicit opt-in, that's good enough for us!
For more information on how users can opt-in through different growth tools, go here