That's actually precisely the problem. People abusing the system make things more difficult for people who have legitimate reasons to be the exception to the rule (e.g. people with allergies).
That's a problem, but it's not the problem. I'm not sure why you guys are acting like it's an unreasonable suggestion for a sushi restaurant to serve sashimi.
I don't think that anyone is objecting to them serving sashimi. People are objecting to people throwing out most of the sushi rolls and only eating the fish. Both because doing so is wasteful and because it appears to be willfully abusing an unintentional oversight in a way that hurts the people running the business.