The legal trouble with public domain is the question "how did it get here?" In the case of expiry, that's simple: you can no longer assert control over the work- it's too old. In the case of a deliberate admission of release of copyright, how do you defend that? If, as a lame example, I take something that is public domain and claim it as my own work, how do you demonstrate that it's not? Because I'm not the original author? But that means that the author is still exerting some sort of implicit right, even if they don't pursue it themselves. This isn't a natural way of thinking about ownership, but it is a legal problem and so some organisations shy away from hard PD.
The fact that you are not the original author has nothing to do with the author exerting any control, it is a question of fact. Beethoven no longer exerts any control over the Moonlight Sonata and yet it is a fact that he wrote it and not you. Why should it be any different for a more recent work released to the public domain by the author?
