Their origin story is Stallman being frustrated with a buggy printer driver that
couldn't be fixed because it was proprietary.
It's told in this talk, you can search for "printer" to find it < https://www.gnu.org/philosophy/rms-nyu-2001-transcript.txt >.
They promote Free Software (and specifically copyleft over permissive licenses) because they view proprietary software as morally wrong and something that should not exist.