Look at the availability of Linux source code compared to Mac OS X. It's everywhere.
In terms of proof and evidence, no one can really "prove" anything without actually experiencing the counterfactual world. You can't prove that MacOS X code would not be everywhere, for instance, just as I can't prove that it would be.
The best we can do is look for evidence that suggests how things might go.
You need to provide evidence that the source of the Mac OS, Oracle or DB2 would be there if copyright didn't exist.