Them not having your best interests at heart does not make them an adversary. Just like you not having the best interests of your grocer or work place or most other humans on the planet at heart does not make you an adversary.
It can be. For example, see "enlightened self interest" (https://en.wikipedia.org/wiki/Enlightened_self-interest), a concept that the companies in question seem to discard in favor of short-term profits.
The Golden Rule "do unto others" is an example of this concept, and it answers your question "why would you?"
> And that is roughly fine (albeit a kinder universe would be nice).
You're confusing the status quo with what must be. It's possible for people to have a sane interpretation of self-interest. We should do a better job of reeducating and/or removing from society the people who don't seem capable of that.
Because the fundamental building block of society (which we are losing rapidly) is some amount of care for your fellow person.
Your comment is a great example of how we Prisoner's Dilemma'ed ourselves into a world of isolation and decaying institutions.
Even if you think of them as the most selfish people possible, billionaires ought to have the average person's interest at heart. Because once the country is full of average people who are unhappy with their lives it typically doesn't work out well for those with power.