They considered it, that's partly why the GPL is drafted as a contract and it is the way is, to be enforceable in countries that accept contracts but may not accept copyright.
There's a similar issue in US law, you could make a case base on copyright ground or on contract grounds. You should typically sue on both grounds (and anything else you can think of) to leave no escape plan to the offender. I believe that copyright is "stronger" and can award more punitive damages so it's preferred, it's all about money of course.
France have droits d'auteurs (authors rights) and contract law. There's also different ways you could sue for different outcomes, the developers went for counterfeiting hoping to seek the most damages (a very reasonable choice). The judge ruled that you can only go to contract court because GPL is a contract and that's it, he pretty much ditched the case.
edit: There's a French principle stating you can't be pursued for both a contract and a civil violation, the contract takes precedence, in a sense by having GPL the developers ditched their right to hold their authors rights. lol
Of course there's another 10 major details covered by 10 other laws that conflict with one another and could have justified any outcome. It's all fascinating and incredibly messy. If you thought they should have gone to the right court, it's not simple, "right court" wasn't a defined variable (and it is still not because the case is going to appeal to try to redefine what's the right court).