Part of the answer is clear enough: combining in order to prevent a market participant from dealing in the marketplace, as in your example, simply must be unlawful...or we're all in deep sheep.
Anti-trust law as enforced is so amorphous, so subjective, that it's hardly any wonder that large numbers of companies and people aren't very informed about it. I'm not, for certain. When you consider the government's sporadic and highly capricious anti-trust enforcement efforts, though, do you laugh or cry?
They pursue IBM for TEN years, then abandon the prosecution. They allege Microsoft is a 'monopoly', or was that 'an effective monopoly', just at the time Linux is becoming increasingly popular, and in doing so cost private citizens billions of dollars by kicking off the tech-share slide (check the date the anti-trust action was announced...the very first day of the tech crash).
If a (presumably) legal issue can't be resolved through the legal system in 10 years' time, then I say the laws pertaining to that issue are either badly written or thoroughly out of date. Maybe deliberately so, hmm?
I think you are a much more informed than most and adding many wonderful comments. Yes, anti-trust law is amorphous and subjective. You are very wise to know that.
I had a graduate business ethics course from one of the smartest men I have ever meet. We used anti-trust case studies as the basis of the business ethics course. The reason was that some folks who did absolutely the right thing were ruined by enforcement of anti-trust laws and some who were crooks were let off. At the end of the course the professor brought us to the point of understanding the cub scout motto, "...to do our best."
As long as one does one's best and acts according to his or her ethical principals, then if you go to jail or are ruined, you will have the satisfaction of knowing in your heart that you were true to your own values. Life does not come with a guarantee that it is "fair." Many of the great tragedies in literature have people who are "tried by the gods" to see if they will be true to their own values or crumble to temptation. The most we can hope for is to be true to our moral compass and prey for the best. The rewards and punishments in life are fickle and unpredictable.
I suspect that many involved with the California power market who are feeling uneasy about the FERC, Justice Dept. and SEC investigations felt the temptation for easy and quick money either for their direct personal gain or for the reputation that scoring big for one's company creates. What I find the most distressing is what I have learned about the ethics and probably anti-trust violations associated with many actions by the Cal-ISO. When the dust settles, I would not be surprised if several Cal-ISO officials don't end up in jail.