Would you feel confident that a person could take a WTFPL licensed software, replace the license with GPL under their own name, and then go out and enforce the license?
Doing WTF you want is still limited by copyright law. You could argue that WTFPL is also an implied copyright reassignment, but I would not bet on it.
Public domain is special since its a natural aspect of copyright law rather than a license, where a person don't need permission once a work is in public domain.
CC0 tries to mimic this but here the tricky part of permissions comes into place. If I give you permission to do anything and you remove the permission text and claim the work as your own, do you still have permission? It a rather popular discussion in regard to cc0, and the consensus where I have seen it seems to be that you need the license in order to have permission. I have also seen lawyers arguing that copyright law itself has limits on what rights a license can give, such as attribution.
