I am not concerned about people using this in non-commercial ways. In fact, I would gladly 'donate' this verse to the Public Domain Foundation if I knew how to do it.
For the record, though, I think you are wrong.
I just heard Pete Seeger's interview on PBS on Labor Day, in which he discussed just this point--namely, that people can take public domain songs and copyright arrangements and variations thereon, such as he did with "Turn, Turn, Turn," which is based on Ecclesiastes.
I have seen many a piece of copyrighted sheet music for old public domain songs, such as "Jingle Bells," "Yellow Rose of Texas," etc.
Ironically, if I am not mistaken, it is only with public domain songs that one can do this.
If you try to copyright a parody, for instance, of a recent composition, still under copyright, you may have to fight in court for the right to even do the parody. (Cf., "The Wind Done Gone.")
Anyway, all I want to do is make sure that if I decide to put this verse on a CD, I won't have to pay royalties to someone else because I failed to put the "©" on it.!