>Tom Lehrer releases song lyrics to public domain

Wrong.

An author (or heirs) cannot put creative works in the public domain. There is nothing in the statute that allows this. Only after the expiration of the copyright does the work go into public domain.

Copyright stays with the author (or heirs) and is subject to license termination after 35 years. This would mean the declaration of "use freely" can be revoked. Which is why these works cannot treated as if they are in the public domain.