nocteus wrote on 07/22/21 at 17:17:15:
how you could violate a copyright on an open source material
The first thing to realize is that publishing something (open source or closed source or other methods) does not negate your copyright. In fact copyright only applies to published works, it's the whole point of copyright to encourage publication by allowing the author to retain the rights. The second thing to realize is that you as copyright holder have the right to charge for use, and can waive that charge ("free" as in beer) under certain conditions. That's where it gets complicated, because everybody understands and agrees with the free part, but they don't understand and agree with the conditions part. So in effect by not complying with the conditions, the user is violating not the copyright itself but the license which governs the free part.
Not giving any details because, first, the GPL is controversial, and second, I am not a lawyer. In particular the GNU General Public License (GPL) places conditions on the use of the software, and thereby on the user of the software, which the average person is going to find strange. As TonyRo points out, people frequently ignore the conditions, but they do NOT have the right to ignore those conditions. It is your right as copyright holder to enforce those conditions.
It wouldn't surprise me if the Stockfish devs have some backing from free-software lawyers with an interest in the case,
pour encourager les autres. There is also the interesting point that by waiting to file, they have allowed ChessBase to ring up more sales, which may mean greater damages to collect. One thing is clear. If they did not sue, history shows ChessBase was just going to do the same thing again in the future.