They have this on their FAQs.
Why do I have to purchase the Pac-Lite barrel from an FFL dealer?
Ruger serializes the upper portion of the .22/45, Mark II and Mark III. Therefore, each of our units, which replace the Ruger uppers, is serialized, considered a firearm, and handled as such.
P.S. They say you can't buy the grip frame and other parts seperately to build with their barrel. You have to buy a whole gun from Ruger.