On being able to fix your own car:
For the same reason that Linux and other open-source core operating systems are the most safe and secure, we legally need the ability to inspect the products we buy for safety and security.
The ability to modify for sport should not be restricted simply because some people aren't capable. Those that are capable should be allowed to, and those that aren't should be allowed to learn.
We don't need more factory workers. We need more skilled laborers, and that requires interest fostered by tinkering. By allowing that tinkering, we are fostering the next generations of engineers. By actively blocking it, we would be fostering highly motivated and intelligent "criminals."
On being able to use 3D printer filaments on a commercial printer:
The reason for the recent popularity in 3D printing, along with a huge boom in innovation and price reduction, has to do with open-source technology. This includes grea strides in materials science.
By restricting the usage of these 3D printers with experimental and third-party filaments, powders, etc., we only slow the rate of innovation, and artificially restrict the sales of a consumable in order to protect the profit margins of a few companies.
In the long run this is neither good for the industry, the country, or those companies. The companies and industry as a whole is harmed because a few companies are allowed to innovate at a much slower pace, while those that would innovate quickly are prevented from doing so.
This then harms the country, because those innovators would then have no choice but to either act as criminals or in other countries that allow such practices. To compound the issue, 3D printing technology is one of a handful of technologies that aid in innovation of other technologies, from industrial machines to household items to medical devices. With 3D printing you have the ability to rapidly create a prototype, and the more innovation there is in creating better and better prototypes, the less work there is in getting a product to market. Standing in the way of that harms everyone.