The recent article down below shows the limited copyright protection of object from the 3D printed shapes.
The article concerns a lawsuit where Inhale sues Starbuzz for copyright infringement. Inhale says that Starbuzz copied the original hookah without permission from Inhale. But Inhale had a problem, copyright can only protect non-useful object. According to the judge, a part of the hookah is non-useful (he describes the shape as some sculpture) but an other part useful: you can inhale the content within the hookah’s shape. For the judge it wasn’t clear what the purpose is of the hookah: the useful or non-useful part. So he made he made clear that the copyright protection couldn’t be applied.
If you look at this case you now many similar cases like this will follow. This shows that there is a problem it the copyright protection and that there is a need for some small changes. Just like my source said: “Would a person 3D-printing and selling hollowed out versions of the original solid sculpture as hookahs be infringing the author’s copyright?” They shouldn’t, but according to the law they will.