One of the manufacturers is Cassina, this is what their website says about the piece.
"All the models in the Cassina collection, by merit of their artistic content and particular creative character, are protected by copyright, a legal institution that is universally recognised and safeguarded; legal protection is assured for the whole life-span of the author and for 70 years after his/her death (or the death of the last surviving co-author)"
Surely this does not mean it is illegal to sell a Hill House replica?