Hello Marius and Vincent,
The access to changing access rights is a topic I recently looked at as well, and a ticket is open on this topic as https://jira.xwiki.org/browse/XWIKI-2184 (but as a new feature request).
As Marius said, it’s been like that forever. The only change that may have happened is that it’s possible that the requirement of the admin right for changing the rights on the space was hardened in version 5.x of XWiki, when the new security module was added because that admin right on space was hardcoded in the security module, but I am not sure.
While I agree with the fact that it should be unified (at UI level and at access rights level as well), I strongly disagree that it should be the admin right. It should be a new right, as XWIKI-2184 says.
The problem with admin (and this is the problem that I currently have with setting rights on non-terminal pages) is that you cannot set it at the level of the wiki and inherit it down the page hierarchy, as it has a completely different (and larger) meaning at wiki level, giving access to all administration features.
I will add a comment on XWIKI-2184 to start a discussion there, but it’s very interesting that you brought this up, it’s a topic I am currently thinking about…