A way to think of the agency fee is to think of a bond election on the local level. The voters are offered a choice of whether they want to tax themselves to pay for certain public works. If the majority vote is in favor of the election, everyone pays the taxes, not just those who voted for it. This is considered to be a Democratic process and is widely practiced in the United States. Without unions, employees must rely on the good will of their managers to take care of their needs.
Some people are comfortable in that mode, especially managers.
Others are not convinced that even well-intended management will always take the time to listen to and to understand the problems of the employees. Unions give nonmanagerial workers the right to be heard and to negotiate for what they need. There have been problems with salary, RIFs and the grievance procedure at the Laboratory recently. But like all rights, the effect is negligible if they are unused.