Forcing a few people to pay to represent those who choose NOT to pay is not any more fair than forcing me to pay for political action committees. Too many will ignore the benefits of the union for the math that shows that they can have the benefits without the cost. Only if the unions can manage to add extras (insurance, discounts, etc.) that will further increase dues can they hope to sell their product to the truly mercenary.
Unions have long wanted to force every one to belong and to pay regardless of what the employee might want. Witness their CBAs in states with out right to work laws — they negotiate for provisions in which the employer will force new employees to be members of the union. Of course that's great for the union, but not great for employee freedom of choice. I favor giving the employee the right to choose whether to join or not. That forces the union to convince employees it offers something valuable. That's how it should be. I have no interest in strengthening unions at the expense of employee choice.