🚀 go-pugleaf

The US Copyright law changed recently.  As of March 1, 1989, it is no longer
necessary to have a copyright notice on a work in order to be protected.
This is for works "published" on or after this date.  "Publication" means,
in general, that the work was made generally available to the public, and
is kind of fuzzy in situations like the one described.  However, A could
probably claim that his work was published when he distributed the source.
If this first occurred after March 1, he still has the exclusive rights
to the work, and thus could in fact force B to either pay royalties or
stop distributing works derived from it.  Of course, A might need to go to
court to do so.  In any case, B has no grounds for restricting A's rights -
the original work is either owned by A, or is in the public domain (if it
was published before March 1 without a copyright notice).

The above is my interpretation based on reading various nontechnical copyright 
articles.  If you'd like a more trustworthy reference, try 
"How to Copyright Computer Software", Nolo Press, 1989 ($34.95).