It's behind a preference flag because it's installed automatically, and Mozilla's the only one who can do that. That's also why there's a lengthy approval process before such an addon can be pushed out:
You can just not install (and enable) addons automatically. Those addons should only be delivered to users who have 1) opted into the study process and 2) opted into the study in the first place.
https://mozilla.github.io/shield-studies-docs/study-process/