What happens when union dues are no longer mandatory?

Apparently, public employees don't rush out to pay union dues when they are given the option. From George Will's piece this week:

And teachers unions may no longer automatically deduct dues from members’ paychecks. After Colorado in 2001 required public employees unions to have annual votes reauthorizing collection of dues, membership in the Colorado Association of Public Employees declined 70 percent. In 2005, Indiana stopped collecting dues from unionized public employees; in 2011, there are 90 percent fewer dues-paying members. In Utah, the end of automatic dues deductions for political activities in 2001 caused teachers’ payments to fall 90 percent. After a similar law passed in 1992 in Washington state, the percentage of teachers making such contributions declined from 82 to 11. . . .

Whether the recent union staff layoffs in Wisconsin is a result of localities no longer buying health insurance through the unions or a drop in union dues isn't obvious.

During the recall tumult, unions barely mentioned either their supposed grievance about collective bargaining, or their real fears, which concern money, particularly political money. Teachers unions can no longer bargain to require school districts to purchase teachers’ health insurance from the union’s preferred provider, which is especially expensive. This is saving millions of dollars and reducing teacher layoffs. . . . .
Democrats furiously oppose Walker because public employees unions are transmission belts, conveying money to the Democratic Party. Last year, $11.2 million in union dues was withheld from paychecks of Wisconsin’s executive branch employees and $2.6 million from paychecks at the university across the lake. Having spent improvidently on the recall elections, the Wisconsin Education Association Council, the teachers union, is firing 40 percent of its staff. . . .