Both are semi-automatic rifles; the difference is that the carbine fires a much lower-powered bullet than the rifle. I don't see that as much of a difference in committing a crime.
There are many, many differences between an M1 Garand and an M1 carbine. The Garand is larger and heavier. The carbine shoots a less powerful round, but has a larger magazine. It was designed specifically for soldiers who wanted a lighter rifle than the M1, like paratroopers and radio specialists. They have something like one butt plate screw in common. The fact that an M1 carbine was used in a crime once 50 years ago---and granted, it was a truly horrific crime---hardly seems like a compelling case for banning the import of a completely different weapon that happens to have a similar name.