Es porque lo que los Estados Unidos llama sidra es diferente a lo que el resto del mundo llama sidra. Tomado de esta ṕagina:
Básicamente, en USA la sidra no es una bebida alcohólica y se le da a los niños, a menos que tenga "hard" en el nombre. Para recalcar lo "inocente" que es considerada por allá, recuerden que Flanders es un fan:The United States
In the USA it is commonly available either "soft" (unfermented) or less commonly "hard" (alcoholic). In its natural state it is an opaque, brownish-red, sweet-tart fruit juice; in this state it can be served as-is (it's a traditional fall drink in this form in much of North America, particularly associated with Halloween and Thanksgiving), spiced (aka "mulled"), or filtered and carbonated ("sparkling" -- a popular substitute for alcoholic beverages for children). "Apple juice" is not always clearly distinguished from soft cider, but is generally understood to mean a form of soft cider that has been clarified and rendered shelf-stable though pasteurization and/or concentration and reconstituting.
The rest of the known universe
It is a fermented drink which is usually a pale golden liquid with about 5-6% alcohol by volume and a pronounced apple flavour, ranging from medium-sweet to bone-dry depending on the manufacturer and the market, and often carbonated. It can be further distilled into a type of brandy known as "applejack", the most famous type of which is the Norman French Calvados. A similar beverage, known as perry, is also made from pears.
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Si Flanders no tiene problema pueden estar seguros que es inocua

Todo eso fue mencionado en su momento cuando los hermanos estafadores aparecieron.