There are a few layers to it.

On the surface, the excuse they give is that printed guns don't have serial numbers and thus can't be tracked. They call them "ghost guns" to instill an element of horror in the messaging.

If you dig a bit deeper, you'll see that it's the same old tactic of manufacturing outrage and spreading fear in order to manipulate the masses. Even when people supposedly see through it, on the collective level it still works. So it seems to be an effective tool, which encourages them to keep using it.

On a deeper level than that, it's a trojan horse. Just one more way to restrict consumers rather than holding corporations accountable. One more way to maintain their hold on people's wallets. 3D printers give people too much autonomy, enabling them to be self-reliant. The people benefiting from the current system can't have that, they won't tolerate it. "No, you'll spend $50 on that action figure, and if you need ten of them you can either cough up the dough or go to hell." Basically.