... well... stepper motors, motherboards and v/t-slot extrusion can be used for many other things than 3d printers...

If they are going to regulate this, then why not CNC machines? Lathes? Drill presses? Pipes and lumber?

Because violent criminals tend to lack wealth, knowledge, and skills. Nobody in the hood about to knock off a 7-11 has a tormach at home and the gcode for a reciever queued up.