Ahem, surely you don't honestly believe your request will be fulfilled with just "one checkbox, just one". It's not the matter of adding
<input type="checkbox" id="justonecheckbox" value="vegetarian">
to the front-end...
Go visit a restaurant in Germany. They have all kind of numbers after their ingredients which show exactly which dishes contain which allergens, as well as showing which options follow popular diets (vegetarianism being the most common one, but also gluten-free, vegan, and multiple others). Why can an online website not do this when a German restaurant with a physical card is able to do this? Its not much work. You just add some more tables in a database with yes/no on certain allergens, ingredients, or dietary propositions. No rocket science, not a whole lot of work. And IIRC San Francisco is quite open minded to alternative diets.
Realistically, not honoring this request would just make those with allergies not participate. That’s a loss of revenue. If OP doesn’t want it, fine, but opportunities are being missed.