Corporations must follow the ethical standard set by the law just as any other legal entity. Corporations actually have more stringent ethics than other legal entities such as persons. A person does not have any fiduciary duty to others unlike a corporation which is incorporated with a set of documents explaining it ethics and responsibilities to other parties (such as shareholders). As a person under the law I can act completely in my own best interest (negligence, power of attorney, and other corner cases excluded) and there is no ethical problem whereas when a corporation puts it's own interests ahead of those of its shareholders it is acting unethically.
I'm not sure we agree on the definition of "ethics".
Ethics, the way I understand the term, isn't a set of rules enforced by law. It's true that some laws are informed by our shared moral values, but in general law and ethics are independent. There is no legal punishment for acting like an asshole, but in human society you more often than not pay a price for it. There is no such onus on corporations and in fact the "business ethics" we govern our corporations by can easily be put at odds with what you and I would consider ethical behavior (see Ford Pinto [1]). As you point out, companies are responsible to their shareholders, but their stakeholders are a different (perhaps partially overlapping) set of people to which the corporation doesn't have to answer. It is in that sense that I said companies aren't intrinsically moral.
