Mares with normal immune function respond to the Montana Solution, so the trait is not passed on to future generations.
Mares with depressed immune function do not respond to the treatments and can pass that trait to their offspring.
Thus, PZP selects for horses with low immune function, which can be destructive to the herds in the long run.
A routine infection could wipe out a population compromised by the advocates.
If the goal is preservation, the use of PZP constitutes a worst management practice.