Car shops are a good example, there is no prevention of repairing other people cars, or their equipment, based on the knowledge from complete schematics / source code. The thing you can't do is make your own version of the thing and sell it.

When my parents wireless router breaks, if I had the source code to the system installed on it I could both better understand the documentation to explain it to them, and I could perhaps patch a security problem that caused the breakdown.

There should be nothing preventing me from re-distributing the fixes, just as Sams FotoFacts used to distribute radio repair manuals, or the Chilton car maintenance manuals.