I don't think that the point of corporate vs public best interest is "unconditional", but we skew way too far to the corporate side than is good for our society
To some extent, an organization is a host for emergent behavior. An organization (like an ant colony) may behave in ways that are counter to the best interests of, and incomprehensible by, any given isolated individual in that organization.
In effect, your suggesting they be double counted as both the owners and members of the public.
That being said, an individual's share of the "public interest" is incredibly miniscule, and there's relatively few owners, so it's not like it's equal on both sides. In fact, that's ultimately the problem; that we constantly bend due to "corporate interests" that are subject to the whims of an incredibly small fraction of society that is, in general, massively disconnected from the norms of society.
Add on to that, generally having a large amount of money and privilege means that most of the things that are in the "public interest" aren't really of significant concern to you because you can simply buy your way into a completely different system (see public vs private education)