If someone put a bagel shop on the Swiss/French border and sold bagels across it (using, I don’t know, a long pole or something) I guess there would be some questions.
Realistically, Switzerland and France have a relationship; treaties, etc. Quad9 has to comply with Swiss law, and they probably hope that making reasonable good-faith moves to block French users will cause Swiss courts (if it comes down to that) to see them as the more sympathetic party.
Nothing's crossing the border in this case other than information. It's more like a guy on the French side of the border shouting over to a guy on the Swiss side, and the French government trying to regulate what the guy on the Swiss side is allowed to say in reply.
Regardless of what analogy you use, the point is that the company isn't in France, it's in Switzerland. It doesn't make sense for it to have to do anything to comply with French law, even if it's interacting with someone who happens to be in France.
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