I suspect this came from the union bureaucracy, who see it as another way to increase the number of members required to run the service. It's probably a disappointment to some, as it could lead to good mileage and overtime payments.
I'm surprised at the difference between how UK and USA (which I have some experience of) union dues are calculated, and the impact on operations. In the UK it is flat per member, which encourages the maximum number of members, regardless of their pay. In the USA it is a percentage of pay (compared to the UK quite substantial), which encourages the highest overall earnings, regardless of how they are generated.