The argument is that you benefit from the union whether you are a member or not because you get the same benefits as union members because of the union's bargaining with the management, so you should pay.
I don't really agree with it, but that's the argument.
Dated sept 19,2018
[..]California Gov. Jerry Brown has signed a law that aims to give public employee unions legal cover from potentially expensive lawsuits demanding that they repay certain fees to workers that the Supreme Court in June determined were unconstitutional.
The law, which takes effect immediately, says unions and public agencies cannot be held liable for fees that unions collected before the Supreme Court ruling in Janus vs. AFSCME on June 27 of this year.
That 5-4 court decision ended a 41-year precedent that allowed public sector unions to collect so-called fair share fees from workers who did not choose to join a labor organization but were still represented by a union. The fees often ranged 70 percent to 99 percent of full dues. [..]