Do you mean unsweetened instead of sugar-free (though that would sound... odd)? Because I know several brands of sugar-free chocolate in Finland that are sold as just that, sugar-free chocolate. They usually contain maltitol, aspartame, or some other sweetener.
Possibly there needs to be something classified as a sweetener, yes, but the definition of "sugar-free" is apparently also anything with 0.1g or less sugar per 100g, so may it be that there is still a bit of sugar in the sugar-free brands you mentioned?
I checked one and it contains 0.1g of sugar per 100g, but it's not added sugar, it's contained in the other ingredients. I don't know if it's possible to make it with absolutely 0g of sugar. But there is no separate ingredient "sugar" there.
