That’s not quite right. As I understand it (IANAL etc), the same rules apply to art/song/performance as to software. The key difference is whether the misbehaviour is carried out by a licensee. Once you license a creative work to someone, whether art or software, breaches are violations of the contact, not the copyright. In contrast, if the copyright holder hasn’t granted a license, then it’s a copyright violation, not a contract breach.

So software does have copyright in France, but it can’t be used for copyleft licenses in the same way as in the US. That doesn’t mean it isn’t copyright.

You say you must release your software under a license. But you don’t have to make everyone in the world a potential licensee. No one is compelled to grant universal license to distribute their software. They can just decline to do so and rely on copyright laws.

You’re defining copyright narrowly as “that which allows open source copyleft licenses like the GPL to work.” But that’s based on a US-centric understanding of copyright that simply doesn’t apply in France.

And it’s not surprising. The GPL is a hack of the copyright system. It uses copyright to do something it wasn’t originally intended to do and is only capable of doing because of the peculiarities of the US (and other) legal systems. Hacks often don’t work outside of their original context.