Pommes de terre, literally means Apples of the ground. Even in English you wouldn't say "apples of the grounds." Or, an other example:

Étrangers de la nuit. -- Strangers of the night. (And not: strangers of the nights!)

@Chris, I think the question is why the second and third time in the text "pommes de terres" is used instead of "pommes de terre." Your response implies that this is a mistake?