That's the point of these systems. If it's not illegal, then it should be.
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To illustrate this, imagine some kind of over-the-top incontrovertible conspiracy to depress wages. I'm talking a secret volcano-island base, a letterhead with a Et redigam operarios in servitutem in latin, members greeting each-other as "Hello, fellow conspirator!" while twirling deliberate Snidely Whiplash mustaches, etc.
Then a new company joins the cartel, and it turns out that company was one of the ones previously paying below the fixed-price.
The fact that some members pay more on joining doesn't change the core nature of the system, especially if there's "noise" in the organic prices.