So years ago, we had GRP (the Gentoo Reference Platform.) My understanding of USE=bindist was that when building packages whose binaries were illegal to distribute, the build system would take some action. For instance, for a while we were not allowed to brand a source build of firefox as firefox, so debian made iceweasel and we ourselves add USE=bindist so we could build custom builds and replacing the branding.
I'm not sure RESTRICT=bindist actually does anything. My guess is that the intention of the restriction is to warn users that when building binaries packages of a given package, there are 'legal issues' with such distribution. That being said, as some have noted in the thread, the legal issues are diverse and are unlikely to be covered in one flag.
-A