>but it's illegal to make certain modifications to it, right?
That's correct in the US. For instance, you can repair an AR-15 back to factory specs, you can modify it in certain ways (adding different sights, etc.), but if you modify it for full-auto operation (which is quite feasible actually), you can be thrown in jail IIRC and be subject to a gigantic fine.
This system works well: people can repair and modify their property to a certain point, but making specific changes are highly illegal, and the penalties are extremely severe, so almost no one does it. The same can be done for other things if the case for public safety warrants it. So repairing your robo-car should be completely legal, but modifying it to blatantly violate emissions laws or to run cyclists off the road can be punished harshly with vehicle confiscation, $1M fines, lengthy prison sentences, etc.
even more, correct me if I'm wrong here, but you can repair a gun, but it's illegal to make certain modifications to it, right?