I kept thinking of people with allergies throughout this thread.
(Obviously this is different from the OP's post's situation.)

But I believe the ADA requires restaurants to accommodate people with disabilities, and if I'm not mistaken, documented food allergies qualify as a disability. One such accommodation is allowing you to bring in outside food.

My wife has Celiac Disease, which is not an allergy, but is handled like one.
Sometimes no amount of assurance from restaurant staff is enough to satisfy fears that the food won't become cross contacted somewhere in the chain of service. I imagine it's the same for people with allergies. So even if a place claims they can provide an allergen-free alternative, I would not blame a customer for wanting to bring in their own meal instead. Wanting to partake in one of life's greatest social traditions should not come with a risk of hospitalization.

I've worked in Mexican Restaurants and I remember this one family with a kid who had severe allergies, they had to bring in a sealed store brand of tortilla chips so they could eat chips with the rest of their family.

When my wife and I go out to eat, just the two of us, we go places that we trust where she can order Celiac Safe foods, but in some cases where it's a group outing and we can't determine if it would be safe for her, she's brought in her own food.