I've hidden my Earth Science community from the "regular" network profile.
But whenever I go on Area 51, it says that I'm active in that community.
Is there a workaround? Why does it not universally hide them in the first place?
As a workaround, you can always create multiple accounts. Jeff Atwood says this, so it should be authoritative. You would just have to deal with losing your rep and not getting the initial +100.