As far as I can tell, the "stats monitor" feature is not advertised on the front page. In fact, I tried navigating the site for a while but couldn't hit the Firefox welcome/upgrade pages that contain the disclaimer.
Most people upgrade their plugin via Firefox's addon manager, and skip the "what's new" page that opens up. This is because we trust whoever made the plugin to not introduce any malicious behavior in future updates.
Judging by the intrusive nature of this feature, I believe it should be opt-in rather than opt-out.