I hear people raise this concern a lot, but I think it is without foundation. If something is in the public domain in the U.S. then anyone can use it for any purpose, including releasing it under whatever license they want. Of course, any constraints imposed by that license will be unenforceable since any user of the software can claim to be using it under the terms of some other license or, of course, as part of the public domain. But if you think you need it licensed, you can have it licensed.