The more complex question may be- why? The answer? To shave a few pennies off the cost of the mass production and slaughter of chickens. According to labels on chicken feed with arsenic, the products claim increased rate of weight gain, improved feed efficiency, and improved pigmentation. In other words, feeding chickens arsenic allows them to grow faster so they may be slaughtered at a younger age; eat less, thus reducing production costs; and have an aesthetically appealing hue when slaughtered. (The European Union has ruled that arsenic may not be added to chicken feed.)
Perhaps the scariest thing about this excessive use of arsenic in animal feed is that all this arsenic does not just disappear- it stays in the environment. As such, even us vegetarians are not safe from the harmful effects of unsafe industrialized factory farming practices.