Similar situation to regular firearms banned by the GCA. The Walther TPH for example is too small to be imported but they can be imported and purchased by LE as like most laws this also includes the "except for us" clause. Once here, they can be sold like any other gun, the only restriction in the law is the purpose of the import must be LE sales.
IIRC, the same thing applied to 89 Ban "assault weapons." I remember specifically the post-89 "Black AUGs" that came in directly to police departments and legally found their way into collector's hands from there.