If you want to give your stuff away to others, you should at least be free to do so. At the moment, unfortunately, you do have to explicitly "license" your content to the public domain, as weird as that should seem.
I did not gave my art away, I sold them.
There is a huge difference between selling art and the concept of putting something into the public domain. Perhaps we are talking past each other here then as well.
As an author and software developer, I take these issues very seriously and it significantly impacts my ability to provide those things I need to survive and to take care of my family. As a result, I have become about as well versed in copyright law as a master electrician would be with the national electrical code. It is the tool of my trade and how I'm able to sustain myself.
When I "sell" a piece of software, I either sell the whole package and everything to it, or I sell a copy of that software. That is two rather different things on a legal basis, as I think it ought to be as well. In the case of those to whom I have sold the entire software package, I am selling all of the "rights" and legal options to that package as well and essentially giving up any control over that software to those making the purchase. BTW, I make most people pay dearly for that option as well and usually charge at a minimum of $50/hour for software that I develop in that fashion, or demand a comfortable salary with generous benefits and a long-term contract. I write the software, but those who are paying for it can do whatever they want including throwing it away and never using it. That also seems to be the story of my life that most of it gets thrown out, but that is a side issue.
If you have a copy of my software, on the other hand, what right do you have to turn around and start handing that to anybody you please? I'm also talking here the difference between an artist who makes an oil painting vs. a bunch of lithographs. Artists that make money off of lithographs depend upon the limited availability of their works again to make a living. Perhaps some other way can be made to pay the artist such as the patronage system of centuries ago, but in a mass market that seems to be the best way on how artists can pay for their time they put into that art work.
There is indeed some stuff that I've done where I've literally given it away, essentially placed it straight into the public domain. Some stuff that I've made as copyleft content, and I've even tried shareware and "postcardware" (just asking people to let me know that you are using it). I've also sold commercial software too. The stated purpose of copyright law is to give incentives to an artist or author (in my case) to have a financial ability to earn an income or perhaps have "other considerations" applied to my work. The "other considerations" with copyleft licenses is that I insist that you "pay it forward" to give others the ability to also share in what I make and you can't hoard it for yourself.
We want people to have exclusive control over the patterns that they acquired. We don't want authors or large corporations controlling patterns you have in your mind, or have special privileges in the market.
We are not against contracts, signed voluntary, that disallow you to distribute it under certain term. However, we are against third parties being inflicted upon by contracts that they did not agree to.
That is the libertarian position.
Nobody is forcing a third party to enter into an agreement with copyleft content. They are free to take or leave that content as they see fit, but if they take the content they must follow the terms of that content. Copyleft is usually a distribution agreement that spells out the terms and conditions for distribution of the content, where as the GPL explicitly states if you choose to ignore those terms then the license is completely void and you have no distribution rights at all. If you say you aren't against voluntary contracts that disallow distribution under certain terms, I fail to see why the GPL is necessarily such an "evil license". If anything, it give more freedom because it allows 3rd parties to have the option to engage in distribution where most other similar kinds of contracts simply prohibit such a practice altogether.
With a "standard" distribution contract, a 3rd party can't use the content other than to simply view it, and even that is heavily regulated. Something like the CC-by-SA license on a movie would certainly permit it to be projected on a huge screen in Central Park in New York City... without even paying a royalty to the creator of the movie. In fact, people watching such a movie could pull out their camcorders and make a complete copy of that same movie, post copies of that movie on YouTube or even press a DVD and sell that movie at a retail outlet. How many "proprietary" licenses allow you to do that? All that the CC-by-SA license insists is that those who buy the DVD and then decide to show it in San Francisco (or wherever else they may go) permit the same ability to others to freely copy the movie. They can't ban camcorders as a form of copyright violation on the subsequent showings. That, to me, sounds a whole lot like liberty and fredom.
I do have problems with EULAs and in particular shrink-wrap licenses, as it really isn't a license at all. Those are things I find horrifying that courts even permit them as they include terms and conditions that can extract monetary damages and basically give a blank check to some of those companies to even perform identity theft and get away with it legally or other things that are really hideous. Again, if you want to find a place to attack, it is something like that which has perverted even the very notion of copyright. Attack the real evil even if you think that copyleft might be the lesser evil.
BTW, I really don't see how this is "the libertarian position", as I know many libertarians who certainly embrace and support the copyleft principles.