It's a legal thing. Everyone is worried that their intellectual properties might turn public domain.
From what I understand unlicensed clean-room reproduction of sets could violate design marks, if those were even registered. Like other trademarks those do indeed need to be enforced.
The infuriating thing about dealing with lawyers on IP is they'll write a C&D that "your work infringes on the IP of my client." There is never any mention of what infringes, except the broadest possible, and there is no responsibility for a false claim.
If that's the case, couldn't CBS have just licensed its IP to these people in a good-natured way?
This is true for silly things like Star Trek to serious things like working with dictators, NSA spying, etc.
And even if this was a concern, you can grant fan licensing, as reddit (see that comment), Lucas arts, most rpg companies, etc have done. In fact, paramount has such a policy, but apparently aren't applying it here.
It really bugs me that companies have convinced people that they "have no choice" but to make people less happy by twisting a law intended to HELP consumers.
That's not the case, and even if it was they still have options.