Tucker, I honestly sympathize with your allergy, but you can always make the choice to not eat at restaurants and prepare your food at home. People with allergies to peanuts have it worse than you; many can't even smell peanuts or be exposed to their residue without being sent into anaphylaxis. Many packaged foods now say "may contain traces of nuts" and that has been a relatively recent innovation to packaging. There is a lot of cross-contamination in food production and it's something that industry and restaurants are gradually becoming aware of. If you have a severe allergy you should always carry an epi pen or two (each one will give you approx. 15 minutes of leeway before an ambulance can arrive if you're in a severe anaphylactic state).
That said, I would still not compare this to outdoor smoking. What happens in private establishments is fine. If a restaurant has smoking, I don't have to go there; just like if a place might have cross-contamination of shellfish, there is nothing forcing you to go there. But smoking outdoors is in public spaces. What am I supposed to do, stay at home? At least with your shellfish allergy, your exposure mostly relates to eating the stuff, and not breathing the scent. You can go to a park and enjoy yourself without worrying about it. Smoke disperses into the air, and I can't stop myself from breathing.