See also "don't use for evil" clause in JSON library by Douglas Crockford and a special exemption for IBM: "I give permission for IBM, its customers, partners, and minions, to use JSLint for evil." [0].
It is a joke but a stupid one that harms adoption of the product. It's conceivable the author could decide say, eating meat isn't "nice" and sue an organization using it that packages meat products for consumption. A judge may find it unenforceable but having to potentially get tied up in a legal battle in the first place already makes it unpalatable to many organizations. See one of the replies here about a similarly juvenile clause where organizations got written permission to use a product "for evil". Not sure a few giggles is worth harming the adoption of technology many people have worked hard to deliver.
