That’s a very good question, but it may have something to do with their past being sort of dark and mystical. It may also have to do with the toxicity of these Solanaceae plants.  The leaves and the stalk of these plants are toxic, but the fruit is not. However, tomatillos and tomatoes are not ‘deadly nightshade’.  Deadly nightshade is known as belladonna, in the same family, but it is not the same plant.