We restricted that because, frankly, Mormon discussions are sometimes weird and confusing to visitors from opposing teams who've never interacted with Mormons before. By restricting it to donors, we make sure that only people who are more invested in the site are part of those discussions.
We also added the politics category a while back, because people tend to get too worked up about politics. There's enough trolling and incivility in it even when it's restricted; I can't imagine what it would be like if we allowed people who just signed up to access it.
The other restricted categories are considered subcategories of one or both of those.