It isn't that the beans are poisonous or something. They contain what the animal nutritionists call "antinutrients."
Heating alters these compounds. Otherwise, they inhibit the digestion of both the beans and other foods. So, the birds are not gaining the full value from what they are eating. They may also become deficient in some needed nutrients if raw soybeans are too large a part of their diets. That can have a serious effect on their health over time.
That is just as I understand it . . .
You can do a google search for "legume antinutrients inhibit site:edu" to learn more.
Steve