So, how are the Ashley's dealt with by those groups?
Fortunately, I just finished reading Our customers are the enemy, a study of cartels in the '80s and '90s, so I can tell you!
Cartels have a number of ways, but the illegal ones have the most problems.
One of the most effective methods was used by Archer Daniels Midland in its lysine cartel: it built lysine plants of grotesque overcapacity, something like 30% of the global market, but only sold part of its peak production; its threat to defectors like Sewon (the Korean manufacturer) was that if they cheated on their quotas, it would unleash a price war that would drive it into trouble (apparently Sewon was very heavily in debt from financing its expansion, and like the 2 Japanese companies, it had minimal non-lysine business) or outright bankruptcy. This is similar to what De Beers & OPEC have sometimes done, IIRC.
Another method in other cartels is the companies shared their internal financial data (whose veracity would be guaranteed by third-party auditors), pooled all the revenues, and then divided accordingly. Obviously this makes it harder to cheat as well, and reduces any incentive.
An approximation to this would be market surveys, and if the surveys showed that any cartelist's share had fallen at the expense of another's, the offender would sell at-cost the product to the damaged party (one of te lysine mechanisms).
Some cartels just hold together because the corporate managers running the cartel have a collective interest in driving up the price & their division's profits, but not much of one to engage in price wars for market share. (Such as the multiple vitamin cartels lasting many decades.)
Others, like the big German conglomerates or Japanese zaibatsu, have been aided by government complicity or active aid.
And then there often can be legal punishments for defectors - going back to the vitamin cartel, we can read in Wikipedia:
"Roche was fined accordingly [for the vitamin cartel], but a bungle on the part of the EEC allowed the company to discover that it was Adams who had blown the whistle. He was arrested for unauthorised disclosure — an offence under Swiss law — and imprisoned."
(And the whistleblower on the lysine cartel, ironically, wound up staying in jail longer than any of the other malefactors because of his embezzlement.)
Finally, one threat in the US that can be used on a defector is that the amnesty program grants complete immunity from federal damages & prosecution to the first cartelist to come forward with good information about the cartel, and this also minimizes their civil liability too - while the other cartelists will still be vulnerable to the triple damages prescribed by anti-trust law, and the civil penalties. So if cheating gets too bad, the irked company can blow the whole thing up.
Related to: The True Prisoner's Dilemma, Newcomb's Problem and Regret of Rationality
In The True Prisoner's Dilemma, Eliezer Yudkowsky pointed out a critical problem with the way the Prisoner's Dilemma is taught: the distinction between utility and avoided-jail-time is not made clear. The payoff matrix is supposed to represent the former, even as its numerical values happen to coincidentally match the latter. And worse, people don't naturally assign utility as per the standard payoff matrix: their compassion for the friend in the "accomplice" role means they wouldn't feel quite so good about a "successful" backstabbing, nor quite so bad about being backstabbed. ("Hey, at least I didn't rat out a friend.")
For that reason, you rarely encounter a true Prisoner's Dilemma, even an iterated one. The above complications prevent real-world payoff matrices from working out that way.
Which brings us to another unfortunate example of this misunderstanding being taught.
Recently, on the New York Times's "Freakonomics" blog, Professor Daniel Hamermesh gleefully recounts a recent experiment he performed (which he says he does often) on students in his intro economics course, which is basically the same as the Prisoner's Dilemma (henceforth, PD).
Now, before going further, let me make clear that Hamermesh is no small player. Just take a look at all the accolades and accomplishments listed on his Wikipedia page or his university page CV. So, this is a teaching of a professor at the top of his field, so it's only with hesitation that I proceed further to allege that he's Doing It Wrong
Hamermesh's variant of the PD is to pick eight students and auction off a $20 bill to them, with the money split evenly across the winners if there are multiple highest bids. Here, cooperation corresponds to adhering to a conspiracy where everyone agrees to make the same low bid and thus a big profit. Defecting corresponds to breaking the agreement and making a slightly higher bid so you can take everything for yourself. If the others continue to cooperate, their bid is lower and they get nothing.
Here is how Hamermesh describes the result (italics mine, bold in the original):
The game? Which game? There's more than one game going on here! There's the neat little well-defined, artificial setup that Professor Hamermesh has laid out. On top of that, there's the game we better know as "life", in which the later consequences of superficially PD-like scenarios cause us to assign different utilities to successful backstabbing (defecting when others cooperate). There's also the game of becoming the high-status professor's Wunderkind. And while Ashley (whose name he bolded for some reason) may have won the narrow, artificial game, she also told everyone there that, "Trusting me isn't such a good idea." In other words, the kind of consequence we normally worry about in our everyday lives.
For this reason, I left the following comment:
I probably came off as more "anticapitalist" or "collectivist" than I really am, but the point is important: betraying your partners has long-term consequences which aren't apparent when you only look at the narrow version of this game.
Hamermesh's point was actually to show the difficulty of collusion in a free market. However, to the extent that markets can pose barriers to collusion, it's certainly not because going back on your word will consistently work out in just the right way as to divert a huge amount of utility to yourself -- which happens to be the very reason Ashley "succeded" (with the professor's applause) in this scenario. Rather, it's because the incentives for making such agreements fundamentally change; you are still better off maintaining a good reputation.
Ultimately, the students learned the wrong lesson from an unrealistic game.