You are asking why courts and society would priority large swaths of regular consumers over that of a few companies in an oligopoly?
Well I guess the reason is because the courts and society cares more about large swaths of regular consumers than they do about a few companies in oligopolies. Thats just definitionally the reason.
> what is the externality
There is no externality. The courts just care more about this group than the other group, and society is fine with that as well.
> Usually there should be.
No, there doesn't have you be. You can instead just care more about 1 group than another. Thats perfectly normal. And a thing many people society do.
I know the basic econ 101 arguments. Here is a pro-tip though. Economics isn't a morality class. Its descriptive. It says nothing about values. One's value system can simply be to care more about 1 thing than another.
It is not weird at all to say that many people care more about large swaths of regular people than a few massively valuable companies, as is the case in basically all market power court cases.