Take Zlib for example. That software is licensed saying (paraphrased):
Keep the authors' names on it, don't pretend you wrote it when you
didn't.
If you change it, say so and don't pretend that we put in the bugs
that you added.
Keep this notice in the code.
If you get a copy of Zlib, that is free, you can run it, study it,
change it, rebuild it and distribute it as you like. MS can also do
those things, but they don't (because it's not copylefted) have to give
you or anyone else their changes. /Their version/ is not free software,
but that does not affect in any way the version you have, that is still
free software.