The default email settings for Phabricator seem to be to mail all the things. This is in contrast to Bugzilla, where a few common scenarios do not result in mail by default:

• In Bugzilla, adding a new CC only emails that person, not all the other subscribers.
• In Bugzilla, you do not get emails about your own actions.

These are both sensible defaults, and Phabricator should work the same way unless a user opts in to getting mail for those cases.

From the preferences, it looks like maybe it's not possible to just get email if someone adds *you* as a subscriber, but not for all other subscriber events. If that's not possible, maybe that can be an upstream bug for Phabricator.org?