French supermarkets all have very large and wide selection of "free from" style products for allergies and intolerances - gluten free, milk free. Plus vegan and vegetarian. In fact it would put most supermarkets in the US to shame. I also expect that many cafes in Paris cater for people with allergies.

So it's not that. More likely it's a surly waiter, or a tourist asking for something which is not on the menu and being upset by the answer.