not to mention the idea that you can somehow know about all 'bad' things that could be created with a bio-assembler in order to forbid them in the first place.
the only way to make it impossible for such devices to create something 'bad' is to make it so that they can only create things that are known to be 'good', and then make it impossible to modify the devices (not just make it illegal, because people deliberately trying to make 'bad' things are probably doing all sorts of 'illegal' things anyway, what's one more)