Although the validity of the license could be disputed because of the obscurity of the permission, the legal recourse you'd have is the "estoppel". Meaning if I have a license (and thus the author's permission that goes with it) to act, and the author later sues me for something that is already permitted, the estoppel defense is raised to dismiss the claim.
But the author hasn't clearly granted me any permissions. He just said to do what I want to. Which is what I do anyway. So why bother saying it?
Even if we can construe "do what you want to" as granting some kind of permission, it isn't clear what permissions it's granting. It can't really be that I have his permission to do literally anything I want to. That's not a reasonable contract. Since it appears to permit everything, it also permits things which the author would easily and rightfully be able to sue me for, which the author can't waive. So it doesn't permit just anything I want to do. But that's what it says. So what does it permit? The hell of it is that there is no way for me to know until I'm already in trouble. The language of the license doesn't specify any class of safe acts regarding the code, some court has to decide what it allows. And I really have no way of knowing whether what I'm doing with the code will trigger a suit or whether the court will decide it wasn't permitted. While estoppel could be worth trying if you got into this jam, it isn't clear that estoppel will save you as a user of WTFPL code. And if it isn't clear to me, it won't be clear to an author who licensed a project WTFPL and wants a legal remedy against me for some use of his code; even having an issue go to court threatens my livelihood pretty effectively.
