In the eyes of most people, the share-alike clause that limits the freedom of image publishers in certain ways is believed to be necessary for the greater good. That good is spreading data freely and openly. The hope behind this is that freely distributing data will compel others to contribute improvements in kind so that everyone can benefit from the end product.
The share-alike license allows users to do what they want with a file's data, as long as any modifications or additions are made under the same free license. Others view this property as an undesirable reduction in freedom, preferring that contributions be put under a free-for-all or Public Domain license. There are numerous pros and cons of public domain clip art, but it all comes down to your understanding of what constitutes public domain.
Even if you declare that your contributions are public domain, realize that you're only making a statement. You won't be able to change the license or what people can do with your data because database law will override individual items. It all comes down to the website you use to distribute content. Even if the majority of the content on a site wants to be public domain, if it isn't in the original structure, it won't be true of your content.
Public domain is the idea that pieces of work can exist without original authors claiming any rights to them. For example, this could be any number of stock photos that a photographer takes and decides to share freely with the community for whatever use they may be.
How to Declare
Depending on the jurisdiction, you can create something and place it in the public domain on your own. In other jurisdictions, your rights as an author simply cannot be signed away. With these cases, public domain will only be used in the sense of "I am still the author, but you can do what you want with my work."
It is still your choice to decide if a work you create will be in the public domain.
Public domain is simple for both users and providers of data. No one needs to be concerned with legal implications, though that will still be debatable between jurisdictions.
The goal of public domain is to be fair to both providers and users. Releasing data into the public domain allows everyone on the planet to reap the rewards of this available artwork.
Public domain isn't a simple declaration, and copyright will vary dependent on jurisdiction. In some jurisdictions, you'll find that public domain doesn't even exist, and you'll still need to come up with a copyright license or waiver suited to a jurisdiction for giving your work to the world.
This system of data distribution is fairer to some groups than others. In some cases, this would allow certain users or providers to find other data to suit their own ends and then to not give anything back to the community. When thousands of people have contributed to artwork libraries, it can be counterproductive to use that artwork and not give anything back.
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