Yes, they can't, because there is no legally reliable way to do it (briefly, because the law really doesn't like the idea of property that doesn't have an owner, so if you try to place a work of yours in the public domain, what you're actually doing is making it abandoned property so anyone who wants to can claim they own it and restrict everyone else, including you, from using it). The best an author can do is to give a license that basically lets anyone do what they want with the work. Creative Commons has licenses that do that.
