The whole idea of copyleft, invented by Richard Stallman, is an effort to reduce people's ability to limit the distribution and modification of software.
2008 September 20, Berry, David M., Copy, Rip, Burn: The Politics of Copyleft and of Open Source, London: Pluto Press, ISBN9780745324142, OL9409091M, page 137:
Copyleft clearly offers an alternative to the system of private property as res privatae for the developers, an alternative that they perceive as a communal system of ownership that is either shared through an organisational structure such as FSF as res universitatis or through a web of interconnected private property ownerships and mediated through copyleft to create a form of commons, res communes.
1992 May 12, Hemphill, Charles, “status of GNU libraries”, in gnu.misc.discuss, Usenet, message-ID <HEMPHILL.92May12102531@lingua.csc.ti.com>:
What is the status of code linked with a GNU library (e.g., the GNU C library)? I think it used to be the case that the resulting code became copyleft. Subsequently, it was found that this limited the use of the libraries and there was some discussion about lifting the restrictions on the resulting code. Did this ever happen?
2012 January 22, Perrin, Chad, “Clang - what is the story?”, in mailing.freebsd.questions, Usenet:
PCC (Portable C Compiler), meanwhile, spent many years essentially unused except in some of the dustier corners of Unix user communities before being actively developed again as more and more people started wanting a copyfree C compiler alternative to the very copyleft GCC.
2007 May 9, sirblob, “what the fuck is going on? (my 52nd poem of the year)”, in alt.arts.poetry.comments, Usenet, message-ID <firstname.lastname@example.org>:
oh come on folks this is the klf, nobody could be more copyleft than them, the buggers even burnt a million pounds once
2008 October 16, Hewings, Meg, “Copyleft manifesto hits FNC: Copyleft manifesto”, in Hour:
Local filmmaker Brett Gaylor’s doc RiP: A Remix Manifesto is the first open-source film to be made about copyright, and is actually more of an ardent manifesto about why Canada – and the world – should be more copyleft.
2012 October 16, Smith, Darby Minow, “San Francisco artist mends clothes and builds community — just by giving a darn”, in Grist:
But the Free Mending Library is not copyrighted, it’s copyleft. Anyone can start their mending library, anywhere.