> Unions are not allowed to negotiate benefits and wages exclusively for their members.

Parent is talking about "closed shops" where a union uses its bargaining power to ensure the employer will only employ people represented by the union. On the one hand this prevents the free-rider problem you mention. On the other, this is coercive and can erode employees' ability to negotiate about things important to them.

It can also be inefficient-- I've been in workplaces where I got in trouble for moving a desk in my office a few feet, because moving furniture was a closed-shop union job, and I wasn't a member.