How is that odd? The issue was the safety of its French employees, and therefore the order from the French court relates to the Amazon facilities in France.

It seems odd because of the moral hazard. Basically, the court is saying, "Treat your employees as you will, just make sure they're not French.".

Is this really any less of a problem if French deliveries are packed by abused Belgian peons?

No, but unless the French decide that they can force others countries to follow their laws, the only choice is between [French protection laws applied in France; French protection laws not applied abroad] and [French protection laws not applied in France; French protection laws not applied abroad].