They did NOT arrest them for printing or making the lower receivers, they were arrested for suspected identity theft. And according to current laws, they can't be prosecuted for building AR lowers unless they are already felons and can't legally possess firearms or they were manufacturing them for sale or distribution. Making the lowers, having the lowers or assembled rifles, is NOT illegal for your own use anyway. Selling or distributing them would be against the law.

The article says it may not be a 3D printer they confiscated. It is:
http://s3.amazonaws.com/nixle/upload...0319640-media1
Linked from:
http://nixle.com/alert/5403995/
(image 1 on the sidebar)

There is a Dremel 3D printer sitting right there in plain sight (at least the box for it) as well as spools of filament.