Civilian ownership and even carrying of automatic knives is not illegal in all states.
Oregon allows ordinary citizens to own and carry (though not concealed, though Oregon's definition of "not concealed" is rather permissive) automatic knives.
I believe that Florida and Arizona do too, though I'm not sure about that.
My understanding is that some states permit them to be owned by civilians, but not carried in public. This allows collectors to collect and people to retain keepsakes (the knife that Daddy used in Korea, etc.).
Unquestionably, however, there are many illegally-owned and illegally-carried automatic knives out there.
Many people mistakenly believe that soldiers may purchase and own automatic knives personally and carry them any time they wish. This is not true. Only military procurment officers may purchase them and then, only for the official use of their organization.
Most law enforcement agencies do not approve of officers carrying automatic knives since it's very hard to justify them (especially in the light of the many fine one-hand-opening knives available today) and they really don't play well in court.
So, hopefully that gives you some understanding of the laws surrounding automatic knives. If you're scrupulously (and there's a good Scrabble word for you -- three u's) legal, you should consult with your local authorities about the laws in your area.