Without defending this action per se, I think it's pretty clear from the number of unserialized guns being recovered that criminals are using 80% receivers to obtain guns.Eagle Arms really thinks that average street criminals have the patience and skill to finish milling a lower receiver when they can more easily steal a complete firearm?
Now, whether that's a reason to ban them... not sure I agree with that. We're at the point where you can just flat-out 3D print receivers/frames now, and even Glock frames only require a simple rail component to finish. In a few years, CNCs that can mill out a receiver from a block of aluminum or something only roughly forged will be widely available. Closing the barn doors now is a waste of time.
That said, their show, their rules. If they think the potential liability from selling 80% receivers exceeds the benefits of allowing them to be sold, hard to blame them from banning them from a business perspective.