I think the only way corporations can reliably act morally is if that is in their best interests. PR, in other words, has to have monetary value, whether it be the appearance of morality to the customers, or an internal increase in their ability to hire and keep good workers. If customers and workers judge a company on its ethics and take their talent and business elsewhere, that will drive the market.

Edit: I think this is also an argument for unions. If a group of workers all agree to hold firm on an ethical standard and block the talent pool, that has power. Without this layer of organizing power, there will always be someone willing to subvert the ethical standards for a quick buck.

Corporations are just a reflection of their owners/management. There are some of them in the hands of good people (e.g. Patagonia). It's hard for them not to become profit-maximizers, but not necessary.