What's the purpose of it? To mark those pictures that were censored or that have censored versions also stored here? If so, shouldn't they just lack the censor tag and be parent/child tagged? Or am I missing something?

I never understood it either. :\

Posts that have been uncensored get that tag.

Then shouldn't they all be parent/child related to their censored version? Most of the pictures Is ee in the uncensored list don't seem to be related to their censored versions.

maybe it should be 'decensored'?

That would work, and better than anything I could suggest, I think.