One can only guess that the French were not dummies, and that they created some corporate veils between their US entities, and entities that dealt with Iran. Therefore, one can assume that there was here an issue of jurisdiction.
One can also assume that there was here some violation of French law, which the French government refused to prosecute... (the French are notorious)
Regardless, as in the Bank of China case, the US is simply trying to extort, based on its dominance in international clearance transactions.
However, this approach, by a nation led by a banksters-security gang, is short sighted.
It is likely to lead either to the demise of the USD as an international trade currency, or further reliance on clearance services outside the United States (particularly in Europe), or both.
Anyway you look at it, it would hard the US economy and international standing, beyond being an act of international hooliganism.