I mean, imagine telling someone who was once convicted of a crime "no sorry, you can't buy a 3D printer to make things around the house, you could make a gun as well!"
If the only thing a 3D printer could do is print a gun, then there'd be an argument. This is like banning callipers from convicted criminals because they could be used to measure ammunition. If this law somehow sticks, I will be very disappointed in the shortsighted thinking that led to such a law passing.