I think the only reason has to do with capacity.
If you were to establish a registrar's office that would be responsible for maintaining the database and some information on the activities of all those seeking to impact Canadian policy and outcomes and so forth in a legitimate fashion, the amount of information would simply be overwhelming, to the point that it would—as a senior colleague of mine, Wesley Wark, refers to it—have the potential to develop into a form of almost security theatre, where you have an office that exists and that by trying to watch agents from around the world simply cannot do so.
The idea is that if you were to focus your attention on updated and rolling lists for which there was evidence to suggest we should be focusing on certain regions, you would have more capacity in that institution.