Where in the document does it actually grant you copyright license to the source code or other IP in question? The only language that's close to a grant of license is:
"Everyone is permitted to copy and distribute verbatim or modified copies of this license document, and changing it is allowed as long as the name is changed."
Notice nowhere in the WTFPL is the source code, documentation or any other IP other than the license itself actually mentioned. Compare this to the grant in the MIT license:
"Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:"
Of course you could argue "well, everyone knows that it's MEANT to grant license to the source code if you put it in a LICENSE file" but in my experience, lawyers encountering the WTFPL have pretty strongly disagreed with that stance.
"TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION" pretty clearly applies to the package accompanied by the license. I looked at the Wikipedia page and this wasn't raised as one of the objections. Do you have a link to some lawyer complaining about that? If the OSI lawyers said it's equivalent to a public domain dedication, that seems useful in countries that don't have a public domain.
One of the objections from the wikipedia article you mention: "Software licenses need to give a clear grant of rights to users to be effective, including the right to redistribute and create derivative works. "Do what the fuck you want to" is not a clear license of any recognized copyright rights; the effect is arguably no license at all."
And again, given the paragraph immediately preceding "TERMS AND CONDITIONS.." one could reasonably argue that the terms apply to the license itself since nothing else is mentioned.
This is essentially the argument I encountered when our company was acquired and we had to do a license audit of all of our dependencies and ended up having to change our codebase to remove a couple (fortunately small) WTFPL projects.
