I think that's because, like most geeks enamored of 3D printing, you do not have a firm grasp on what is possible with small-scale manufacturing today.
Even if they switched over to xenon lasers and sintered metal powder, a 3D printer would still be limited to low-force, low-work-energy products. Look at it this way - inkjet printing allows you to print out nifty photos in full color from your house. If you want to print something that lasts, however, you're going to go to a big, professional house that does 4-color offset printing, same as you would have back in 1982. Granted - the Internet has opened up those houses to the rank and file. I can now get a book of my kid's first four months off Shutterfly for like $20. But as far as the actual product made, better results are available with less effort to trained professionals with traditional tools.
I first messed around with 3D printing in 1997. It's awesome for rapid prototyping. You make your thing into an STL file, hit the glue, and then cast it up using lost wax or sandcasting and you have a real part.
Lost wax and sandcasting have been available to the home experimenter for over 10,000 years. But nobody messes with them because they require skill.
This little gun basically makes it so that goofuses with a $2000 printer no longer have to buy a piece of pipe and follow the instructions in the Anarchist Cookbook to make a zip gun. They can just print it out. In the end, though, they're both going to run afoul of a metal detector... and the piker with the Anarchist Cookbook will be able to fire more than 4 shots out of his saturday night special.