When one party lies to, deceives, or otherwise coerces another party in the interest of getting that party to part with something of value, that is fraud. A business that injects poison into its food in order to reduce costs and subsequently lower prices so consumers will purchase their products, it is essentially a party coercing another party into parting with something of value, which in this case is money.
So, if you think fraud should be illegal, then, from what I understand, you should also conclude that businesses should be required to indicate to their customers that the products they are selling are poisonous. Personally, though, I don't think fraud should be illegal because I don't think anything should be illegal. Legality implies the existence of a state.
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