The page does not list enforcement mechanisms for cartels, and in fact says: "In general, cartel agreements are economically unstable in that there is an incentive for members to cheat by selling at below the agreed price or selling more than the production quotas set by the cartel (see also game theory). This has caused many cartels that attempt to set product prices to be unsuccessful in the long term. Empirical studies of 20th century cartels have determined that the mean duration of discovered cartels is from 5 to 8 years."

Come to think of it, the general idea is "If you, as an insurance company, foot the bill for these treatments with long-term benefits for your clients, this will cost $X, but save you $Y and other nearby insurance companies $Z in the future. Therefore, we the association will pay you $z < $Z to do it, and by assumption $Y+$Z > $X, so there should exist values of $z that make this profitable for both parties (anything between $X-$Y and $Z)." The carrot is the payment of $z.

In terms of whether this association could become a price-fixing cartel... It's probably easier to verify compliance when compliance consists of performing a positive behavior (i.e. paying healthcare providers to do certain treatments; whatever the details of the payment, the important thing is whether the treatment was performed) instead of a negative behavior (i.e. not selling extra product at a lower price on the side). It doesn't seem obvious that this would make price-fixing much easier than it currently is.

Or this one: https://en.wikipedia.org/wiki/Central_bank