Update (2009-12-09): Via @miraglia, here’s a hilarious excerpt from Doug’s talk, “The JSON Saga”, in which he gives some background on why he added this clause to the license and how often people ask him to remove it:
When I put the reference implementation onto the website, I needed to put a software license on it. I looked up all the licenses that are available, and there were a lot of them. I decided the one I liked the best was the MIT license, which was a notice that you would put on your source, and it would say: “you’re allowed to use this for any purpose you want, just leave the notice in the source, and don’t sue me.” I love that license, it’s really good.
But this was late in 2002, we’d just started the War On Terror, and we were going after the evil-doers with the President, and the Vice-President, and I felt like I need to do my part.
So I added one more line to my license, which was: “The Software shall be used for Good, not Evil.” I thought I’d done my job. About once a year I’ll get a letter from a crank who says: “I should have a right to use it for evil!”
“I’m not going to use it until you change your license!” Or they’ll write to me and say: “How do I know if it’s evil or not? I don’t think it’s evil, but someone else might think it’s evil, so I’m not going to use it.” Great, it’s working. My license works, I’m stopping the evil doers!
Audience member: If you ask for a separate license, can you use it for evil?
Douglas: That’s an interesting point. Also about once a year, I get a letter from a lawyer, every year a different lawyer, at a company–I don’t want to embarrass the company by saying their name, so I’ll just say their initials–IBM…
…saying that they want to use something I wrote. Because I put this on everything I write, now. They want to use something that I wrote in something that they wrote, and they were pretty sure they weren’t going to use it for evil, but they couldn’t say for sure about their customers. So could I give them a special license for that?
Of course. So I wrote back–this happened literally two weeks ago–“I give permission for IBM, its customers, partners, and minions, to use JSLint for evil.”
[laughter and applause]
And the attorney wrote back and said: “Thanks very much, Douglas!”
You can see the full video of the talk at YUI Theater (the excerpt above is from 39:45).