I always assumed it was more about what you do elsewhere in life once you've invested in an "unethical" company: If I'm invested heavily in the lemonade stand,

I always assumed it was more about what you do elsewhere in life once you've invested in an "unethical" company: If I'm invested heavily in the lemonade stand, it's against my interest to point out that too much lemonade could give you diabetes. So the point is more to free yourself from an obligation to act immoral.