It's not greed, just survival. For some unknown reason antibiotics have a synergistic growth effect on animals that are not sick so antibiotics are feed to healthy animals. In the real world most businesses are barely profitable so any action that can increase profits is used to avoid bankruptcy.
Horses**t. The first farmers who did this did it because of greed, trying to make a bigger profit. Later farmers might have felt that it was the only way to survive, but only because the first farmers did what they did.
If your business isn't making a profit, you raise prices until it does. If you can't do that, it means either that you're doing something inefficiently or that somebody else is cutting corners. If it is the former, you need to fix the inefficiency. If it is the latter, you need to clearly differentiate your products from those others in the marketplace so that your customers know why your products cost more. Either way, cutting the same corners that everybody else does invariably results in a race to the bottom, not just in terms of cost, but also in terms of profit margins and quality. Once your business goes down that path, you might as well close the business and give the money back to the shareholders, because it is a hopeless cause, and your business is no longer contributing anything of value to the world as a whole that could not be contributed just as easily (and more efficiently) by your competitors in your absence.