I've asked and asked and asked and asked and still can't get a straight answer, as to the origin of this belief or superstition among most of the Chinese chefs. But, they refuse to prepare green beans, because they are poison and they don't want to be responsible for making someone ill by serving poisonous foods. There's no real rationale to support this beliefe, but it's what they have been taught; I don't understand it, but I respect the country and culture and, therefore, respect their beliefs, including their belief that green beans are poisonous.
They also wont prepare Swiss chard and some other fresh, leafy greens. Either they have never used those ingredients and don't want to try or they have some superstitions regarding them much like the green beans.
I will admit that there are ingredients I will not miss and will not use in my cuisine when I return to the United States. Wood Ear Mushroom, for example - yuck!!!!