I have read the ATF has issues with adding bayonets to SKS's that don't have them and with taking them off SKS's that do have them. Make sense to a bureaucrat I guess.
Because they were imported as curios/relics based on having specific features. Typical stupid law that should only affect (if anyone) an importer if he violates it, but is used to scare regular folks who just want to modify their own personal property to their liking.
