Dunno but it doesn't even matter. You can enforce your copyright whenever you want, that's the rules of the USA. You don't have to enforce it all the time if you don't want to.
Names aren't protected by copyright, and its far from clear that a spec of this type is a non-fair-use derivative work such that it would require a license from the copyright holder of markdown.pl in the first place.
The license only applies to derivatives of the markdown.pl code. The spec describes a "reverse engineered" implementation that is partially interoperable with the original but isn't actually derived from it. Reverse engineering for the purpose of interoperability is a legal activity in the US.
