You can’t make and sell copies of (say) Mac OS X without Apple’s permission. But you can use TextEdit (which comes with Mac OS X) to write a text file, which you can save, copy, and sell without further obligation to Apple.
Question: how do we know we can do this? Where is it written?
Similarly, how do we know, when buying a typewriter, that its manufacturer doesn’t acquire some rights to whatever we produce with the typewriter?Email me.