Well that's because diet and regimens are very different on either side of the Atlantic.
This side of the Atlantic, regimens would more often be use to define your exercise schedule, not your food intake. Diet is usually a plan for food intake, especially for limiting the same, or for some unusual habit of food intake ("Her diet consisted exclusively of twinkies and diet coke!") So a well balanced diet wouldn't be carried to you on a tray, especially not in a restaurant!
What you want to be balanced on the tray is Food! comestibles! libations! A restaurant is no place for a diet!


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