Eh, every potential customer that believed the lie about their food wasn't going to be a repeat customer anyway. Because anyone too stupid to get both sides of the story is the kind of idiot that is absolutely going to die in the labyrinth.
Well, the writer isn't all that bright anyway. Bet if they said rice is grass seeds then the adventurers would be weirded out even though someone in such a society as theirs would be fully aware that wheat and such are grasses and so would see that as perfectly normal.