That's what I meant, the previews are on by default for not-logged-in users, but appear to be off by default for logged-in users (or at least the preference was set to 'off' for me).
Perhaps you had enabled the beta feature and when the feature went to production the setting was kept enabled? Check under Preferences>Appearance>"Reading preferences".
Hmm, that could be it, since it's showing up as enabled for me. I have "Automatically enable all new beta features" on, so I guess I was just betaing this feature for a long time.
