Quest, with all due respect, it may be as simple as you can get, but that doesn't make it a fact. Using a purchased or free 3d model that explicitly states you can use it for commercial or non-commercial use, without specifically crediting it, is perfectly legal & is not copyright theft. It may offend your own personal sense of ethics, but that doesn't make it illegal.

3d models that are made available by their creators, whether for free or for profit, are designed to be used by others in works of art - that is their purpose. That's very different from say, the copyright  case brought against Shepard Fairey for his use of an AP photo as the basis for his famous Obama poster. In that case, a copyrighted photo that the photographer did not intend or explicitly state could be used by others was used as the basis for a poster. Even that case is, as far as I know, still not settled conclusively. If the photographer had sold the right to use the photograph in a commercial work & had not specifically stated that he desired to be credited (as is very standard practice with stock photography, which is probably the closest parallel to the use of 3d models), I believe he would have been laughed out of court.

I have to keep restating that I do appreciate the work of model creators & often credit them. The fact that not tediously mentioning every single model I use every single time I use them doesn't meet your particular personal set of  standards does not make me a copyright thief - it just makes you judgmental. I'm really not looking to start a personal confrontation here, but I don't appreciate being called a thief. I would hope we can agree to disagree respectfully on something that is clearly not as cut & dried as you tried to make it sound.

"Every child is an artist. The problem is how to remain an artist once we grow up." - Picasso