You can't do whatever you want with open source either. One big stipulation of copyleft licenses is the share-alike clause, which means you can't make modifications and then decide your program is now closed-source, so it protects the code from being enclosed again.

I mean yes you can make whatever modifications you want, generally, but it's not totally unrestricted.

It does harken back to the original HL mod scene though - some high quality stuff came out of that (and valve picked up the Devs too)