I would say that you fundamentally don't understand what intellectual property means.
The 3D model that I have modeled is the intellectual property, just as a book or a music album is intellectual property. I will explain how it works.
For example, if you buy a book, it comes with a license to do what you want with that book, except for printing and selling it for profit. Same with the music album.
If you buy a 3D model, you either buy it at a cheap price with a standard license, which grants you full right to 3D print it for your private use. This however is not the same as a commercial license to produce and sell a product.
In regards to upholding intellectual property, that is a different matter. As a free market proponent I believe that reputation is the best arbiter in most cases. If I rip someone off, my reputation will suffer.