I've actually *sold* a few of licenses to the public domain SQLite library. Companies call me up and say they want to license the product. I carefully explain that no license is necessary and that they can use it forever for free for anything they want. But they still want a license. So I sell them one. So far, I've sold them cheap. Maybe I should charge more....

This appears to be more of an issue in Europe where, apparently, the concept of "public domain" is less well defined than in the US.