The people who have executive power in the leadership structure of companies control what companies do. When ownership is diversified, business owners
do not determine what their businesses do; your "tautology" is actually false,
obviously so with more than a cursory examination of the world as it is. I myself am a part-owner of ARM, for example, but I have zero effective control over them.
And the question as to what should a company do is very definitely an ethical one. Some people try to argue that ethics only apply to companies in so far as they apply to the people that form the company; but others do not. Because the question isn't "what can a company do within the constraints of law", the question is actually what ought a company do.
Companies help match supply and demand where the supply or demand or both are divided and scattered enough that they can't be efficiently matched through individual contracts. But in this formulation, just like free markets, they have no moral content. The mere fact that there is demand for something does not imply that it is good that the demand is satisfied with supply. Lynch mobs frequently have short bursts of popularity, and they demand someone be lynched; irrelevant of whether they are legal or not, it should be clear that the fact of the demand does not amount to an ethical justification.
So when we ask what ought a company do, it is not enough to look at ownership structures; nor is it enough that we look at the operations of the company in providing supply to market demands. Merely following the law is not enough either; many immoral things are permitted by the law (to the point that later, the law changes), and many moral acts are taxed or prohibited by the law, for various formulations of morality. But more importantly, the law is itself informed by the ethical question over what ought companies do.
For example, it may be the case that child labour is legal in country X, and company A employs children in country X to meet supply for products in country Y, where such child labour is illegal and everybody agrees that it is unethical. By your formulation, there is no way to argue that company A is violating ethical norms. At best, you would suggest that shareholders - e.g. people with pensions, ultimately - should be held ethically responsible. You'll forgive me for thinking that that sounds rather ridiculous.