I've thought about this a bit lately and something came to mind about the commercial renders of free items.  I honestly do not believe this is a legitimate infringement - but whatever Im not going to debate that, lets pretend it is. 

I've known a few people who model during my little stint here, and I hear OFTEN of using RL references for modeling items.  So lets say Joe Generous Modeler, makes say a set of old style boots.  He uses various photo references from around the web to make his brilliant creation.  Then he ever so kindly donates his work to the community to use as a free item - with a small stipulation: 
You can use it, but not in any way in which you will ever make money from taking renders of it.
If you do, then you need to pay him.  Or you simply can't make pictures of his model and make money off your pictures. 

Ok - lets say this is reasonable.
Does he then go and give a portion of his proceeds to the people who made the original pictures that he modeled from?  Has anyone ever seen credit given in a readme for a piece of furniture used as a photo reference?  I've seen 3d.sk credits given and as far as photo references that were not purchased but just found via google images I've never even so much as seen one credit.  Maybe I've missed a few but it's certainly not the norm.

Anyway I think this angle has been completely overlooked.  Or somehow it's ok to copy thiers but not ours - in likeness of course, because a render of Joes model is certain not HIS model.
It is a render of his model -  a model made from a reference of a Sears chair is not a sears chair, it is a 3D model of one.  Where is the line?

I am:  aka Velocity3d