You can cite the one ball rule but that is NOT what is happening here. Mr. WGT made it clear years ago that the goal was to not give players choices.
In real life the one ball rule does not apply to practice. The one ball rule does not apply to low level competitions and social games. WGT should not be the self appointed tournament committee for the country clubs.
Bottom line? You don't get to choose another ball in inventory because WGT sees the dollar signs of forced ball sales and quite a large number of partially used sleeves people purchased and will never completely use. It's all about the money.