Etsy instantly updates people who have been included in a treasury list, as soon as you hit the Save button. Often I hit save if I am just saving a work in progress... This involves a layer of guilt if I later remove someone from the treasury, they've already been informed of their inclusion.
Especially considering the fact that Etsy shows "featured" and "curator" badges on the avatars for those who are commenting on a treasury, its weird to see a post from someone saying "Thanks for including me" and they aren't actually included in the final product :(
It can get.... awkward. Keeping something private until you're ready to show the world reduces these sort of Etsy faux pas.
At least, that's my idea behind private treasuries.