Most countries do not allow their citizens to release their work in the Public domain. But within the restrictions requested by laws, they can express under which conditions they release their work, regarding copying, distribution and modification. This license allows the copyright owner of a product to express clearly that he allows anyone to DO WHAT THE FUCK THEY WANT TO.
Is this license valid?
The Free Software Foundation recognized the WTF Public License as a valid free license. It has been used for publishing some computer softwares. A license is a contract between the copyright owner and the user of a product. Within the restrictions requested by laws, they are free to agree under which conditions this product can be use. With the DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE, the copyright owner express in his own words that he allows any legal usage regarding copying, distribution and modification. It is this authorization notified by the author that Wikimedia Commons record, as any statement made by an author when publishing its own work.
But on Wikimedia Commons, shouldn't we use only a Creative Commons licenses or GFDL?
According to Commons:Licensing, all copyrighted material on Commons must be licensed under a free license that allows anyone to use the material for any purpose. In particular, the license must meet the following conditions:
Republication and distribution must be allowed.
Publication of derivative work must be allowed.
Commercial use of the work must be allowed.
The DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE meets those conditions, with no further restrictions.