Just for the record, other people haven't necessarily seen other comments you've made. Acting indignant about that is frustrating.
What's callous indifference is the company having an attitude that allergy safety is too much work, not thinking you should vote with you wallet.
A lawsuit is part of voting with your wallet. More specifically, giving them a financial incentive to take food safety more seriously.
I seriously doubt the guy is going to go back to either restaurant, so voting with his wallet and not giving them money for a burger is done, and likely doesn't cover the costs he incurred as a result of their error.
When is a lawsuit appropriate if not after a business decides to cut corners and hurts you?