With all due respect I was also born in France and still live there as St Martin is integrant part of it, and you may have your own reading and interpretation of the law, but the law is that law and it says what it says.... Beside that, St Martin's restaurant are under very strong scrutiny and are subject to very frequent controls from the French authorities, I can ensure you that if it wasn't legal, you would not see it on the bill! The restaurant would be fined heavily and eventually subject to a "fermeture administrative" (closing order) if the infraction was repeated. I assume then that I am not the only one to have the right interpretation and reading of the law since the French Government seems to have exactly the same.
Whether "all French people rightfully consider that prices in all restaurants and bars include tax and service because that's what French Law requires" it is, as you rightfully stated, only a consideration and an assumption, I don't...
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