Why is this needed? Please keep this bug open as long as the ebuild has RESTRICT=userpriv.
Actually I had no idea why, but digging in the CVS logs it seems this was introduced because there was trouble when compiling with userpriv and vpopmail support. However vpopmail support got removed upstream a while ago. (We still have an unused use flag for it...) I'll remove the restrict (and also the stale use flag) with the bump to 0.66.4 and with the next stabilization we should get rid of it.
It's fixed in 0.66.4 and stabilization of that happens in #572744. I'll remove the old versions once that's done.