That’s not arbitrary and it’s not ‘free labor’. It’s just business. If paying dues to the union and salaries to the union bosses make sense then the members will pay it. If it doesn’t make sense, then the workers shouldn’t pay it and the bosses have to figure out another way to get paid.
It’s like the owner of an overpriced coffee house complaining he doesn’t have enough income to cover his overhead... what, force the neighborhood residents to stop in once every two weeks?
In most unions, it’s a select group of members doing the collective bargaining, not union employees.