The Free Software Foundation has started to work on Version 3 of the GNU General Public License, better known as the GPL (it's easiest to read Tim Bray's cleanly-formatted version). The GPL, which represents the notion of "copyleft" in contrast to copyright, talks a lot about freedom:
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
Yet GPL'd code is not free as the air, breathable without restrictions. Paradoxically, according to the FSF, true freedom entails prohibitions:
To protect your rights, we need to make requirements that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
In particular, consider the ramifications of the following statement:
Developers that use the GNU GPL protect your rights with two steps: (1) assert copyright on the software, and (2) offer you this License which gives you legal permission to copy, distribute and/or modify the software.
So, in order to protect your rights, a developer first must assert the sole and exclusive privilege to control who may propagate the code and under what terms -- a privilege that is ultimately to be enforced through the monopolistic power of a government. The GPL defines "propagate" as follows:
To "propagate" a work means doing anything with it that requires permission under applicable copyright law, other than executing it on a computer or making private modifications. This includes copying, distributing (with or without modifications), sublicensing, and in some countries other activities as well.
Note also that:
All rights granted under this License are granted for the term of copyright on the Program.
Finally, if you modify a GPL'd work:
The modified work must carry prominent notices stating that you changed the work and the date of any change.
Taken together, the terms of the GPL amount to a complete capitulation to "applicable copyright law", no matter what the content of such law. If a legislative body such as the U.S. Congress should decide, whether in its independent wisdom or under the influence of lobbying interests, to modify "applicable copyright law" so that copyright terms shall extend in perpetuity, the GPL would enforce that. If "applicable copyright law" were changed to require a government-issued license (complete with fingerprinting and other invasive measures) in order to modify or propagate a work, the GPL would enforce that.
As I see it, the fatal flaw of the GPL is step number one: the assertion of copyright. Because the assertion of copyright immediately entangles you in the web of copyright law, it implicates you in whatever copyright laws should be promulgated or modified by governments anywhere in the world. Because copyright terms have been extended continuously by governments, the GPL logically implies an acceptance of perpetual copyrights and therefore logically prevents GPL'd works from passing into the public domain of the common stock of knowledge and craft (and we are not far from converting that "logically" into "physically", I think). The GPL cedes the field entirely to government law-making, rather than basing its contracts upon natural law or the free marketplace of ideas (a lex mercatoria for the digital age).
The great worry behind the GPL and other such licenses is that someone will modify your work and fob it off as the original. Yet the answer to that worry is not restrictive licenses (viral or not), but the very act of publication: make your work public, sign it, date it, identify its author, establish its canonical URL, and loudly proclaim its existence to the world. If anyone else should dare to assert ownership over your work, the public record will make it clear whose claim has priority. (And, I think, the resulting publicity will only redound to the fame and repute of your work.)
So-called copyright is not an individual right but a government-granted privilege to control who may propagate ideas. Let not the lofty rhetoric of freedom obscure the basic fact of the matter: copyright is all about control (enforced, ultimately, at the point of a gun). A copyrighted work is not free as the air; only a work in the public domain is.