Consider that Local Motors themselves said the cars are not street legal.
'Street Legal' can mean many different things. In many cases this would effectively be a 'hobby car' - IE built by the owner, one-off, etc... In many states making one street legal is around a 10 item checklist - does it have brakes? Does it have functional brake lights? Can it turn? Functional turn signals? Windscreen, headlights?
Safety of the occupants of said vehicle is not really addressed, just that they aren't a rampant danger to the other people on the road.
Emissions can be tougher.
It ends up on the definition of 'manufactured', they may be utilizing a loophole which even auto makers exploit for things like their concept cars - one off cars are crafted, not manufactured. Ergo exempt from everything, but they have to actually be 'crafted', IE hand built in a one-off fashion.
With the 3D printer system they could be leasing the equipment and assistance to the buyer, who actually triggers the machine. Since his modifications make the vehicle one-off, built by him(technically), it's not 'manufactured' under the definitions.
Much like how you can buy a complete kit car that's also exempt. You just have to put it together.