I think that sort of policy does have a rational basis. If one organist is willing to work half his hours for no pay, it creates an incentive for the parish to squeeze out the other organist. Working for that parish can become a race to the bottom.

My son is the only organist at our parish and listed on the church bulletin as such. There are three Masses each weekend; my son plays only the 10:00 a.m. Mass on 1st and 3rd Sundays, and 5th if there is one with an occasional Feast Day thrown in. His volunteering to play the 8:00 a.m. Masses would financially impact no one (though it would cost our family to drive an extra 60 miles round trip each Sunday and require a significant sacrifice of time which we would have offered gladly to correct the abuse). But it would bring positive good to the Mass.

I am not arguing against the policy for the examples mentioned in large parishes with multiple paid musicians. What is troublesome is that such policies can be used against smaller parishes like ours where the intent of the policy is moot. So our parish continues to suffer with wretched recorded music because of a policy that doesn't even apply in our situation. Why can't those who think up these things consider everyone involved, not just the big guys? They could, but they won't, and I know why. I've seen the same effect in many other policies that have come down from the diocese.