On the one hand, I agree with you.
On the other hand, if you're deathly allergic to something as common as onions, you probably shouldn't rely on fast food workers to keep you alive.
I've got a friend with actual Celiac's disease. To the point where a drop of wheat could be the end of him. He does not take this kind of chance, ever. He trusts me to cook for him, but I care about his existence beyond just being a customer.