Links from registered users have the nofollow tag. But if you use the unfurl link with preview the tag is missing for all usergroups and all external links are dofollow.
I think that's a bug and it should be solved.
Furthermore in my opinion it is not acceptable that staff external links are dofollow by default. In a community with many moderators there is to less control about external links. We need a setting that prevent dofollow external links by default for staff.