It's what is happening now because that is where the interest for home 3D printing is and what the capabilities of those printers are. 3D printing is popular a

It's what is happening now because that is where the interest for home 3D printing is and what the capabilities of those printers are. 3D printing is popular among the maker crowd, and the printers are mostly capable of doing relatively low resolution, single material prints of models specifically designed to be printable (low overhang, fitting within a small build envelope, no features smaller than a couple of millimeters, etc). I suspect it won't be that long before home 3D printers are much more capable and there is greater interest from commercial content producers to access that market. Imagine for example a functioning single piece carburetor. Some amount of engineering would go into such a thing, and it would be non-trivial to reverse engineer a copy of it by scanning or other means. It would however be trivial to redistribute the file on the Pirate Bay. I think what we will see at minimum is objects being sold as signed files containing "compiled" G-code (or some proprietary equivalent) for each specific printer model rather than as solid object files. The G-code could always still be reverse engineered based on the commands arriving at the motor controllers, but it would be less easy to adapt to arbitrary printers.