There is No Legal Mechanism to Put Something in the Public Domain

This article is slightly misleading. It says "Under basic contract law, a gift cannot be enforced."

The promise of a gift cannot be enforced. But, once a gift is delivered and accepted, it cannot be taken back.

If Developer Dan offers his software for free, to use as you wish, no strings attached AND you download and use it (clearly "delivered and accepted") he cannot decide later you can't use his software.

He could decide later he wants to stop giving it away (to new people). But even in that scenario, YOU have a reasonable argument that YOU are entitled give his software away to anyone you wish.