And the suppliers will say "good-by, I'll make stuff for Widowz and Android instead. If you want us to meet these conditions, you (Apple) will have to pay us more for our products to cover the cost difference".
Making the assumption that they would find other customers. Imagine Greenpeace saying "why is company X using Y as a supplier, when Apple cancelled their contract with Y because they are exploiting their employees and destroying the environment"?