It's an interesting question. My understanding is that the artist's copyright of a model prevents anyone redistributing or copying the MODEL, but that the only copyright in an image is that of the creator of the image. Just as if I take a photo of your house, I don't infringe any copyright on your house. A model used in a render is analagous to paint used in a painting, and the paint manufacturer has no rights in the painting. Secondly, so far as I understand, downloading a model and using it does not constitute a contract with the maker of the model. The modeller may set out terms and conditions but do these have any legal force if there is no agreement with the user? I suspect not. Obviously it makes sense to respect the modeller's wishes, or one would be biting the hand that feeds one, but if one wishes to view it from a legal angle, the case is far from clear.