...police seize 3D printers...
Well, they should also seize pen and paper because they are used for writing bad and violent notes by the bad guys. And all those paper planes blinding us when they are colliding with our eyes. It would be good to lock these dangerous materials away from general use.
Next we will all need a weapon license to buy a 3D printer and an extra fee is put on the raw materials. You are only allowed to buy raw materials with the license and the raw materials will be DNA marked with the license ID for tracking purposes. The collected money is for the copyright infractions every owner of a 3D printer surely is making all the time. The license ensures that we will kill each other in a registered and trackable way. This setup will work magic because only criminals will ignore the rules and will therefore be caught.
I'm wondering when someone invents 3D printing explosives for human consumption.