Myth: Free Riders and
the Duty of Representation

Reality

Unions do not have to represent all workers in a unit, nor do they have to represent non-members.

It is perfectly legal for unions to organize "members-only" unions, and, in fact, sometimes they do. Members-only unions used to be more common in this country, but once the National Labor-Relations Act changed the rules of the game, they largely fell by the wayside.

Even now though, members-only unions exist. They are arguably even seeing a resurgence, as labor unions seek to gain footholds in workplaces where they functionally don't have any other options. A relatively prominent example more recently would be United Auto Workers (UAW) Local 42, which was formed at the Volkswagen Assembly Plant in Chattanooga, Tennessee after the UAW failed to win a majority vote of the workers.

However, if you're a union boss, the problem with a members-only union is that your power is pretty much limited to your membership. That doesn't sound unreasonable in virtually any other context, but American labor unions have enjoyed a century of unique legal privileges. 

And the fact is, monopolies are powerful. Therefore, instead of members-only contracts, modern unions will almost always attempt to impose monopoly-bargaining contracts, using federal law to force all workers to submit to their so-called representation. As you might expect, these monopoly bargaining contracts give the union boss a monopoly over bargaining in the workplace -- that is to say, they remove the ability of anyone else to bargain for themselves.

If a union boss chooses to impose monopoly bargaining authority over a workplace, then and only then do they assume the responsibility to "represent" everyone in the workplace -- because they have stripped that ability from everyone else.

However, at no point is the union forced to take on monopoly bargaining authority, and therefore at no point is the union forced to represent non-members. No federal law imposes such a requirement on unions -- only their own contract language, which they have chosen themselves and fought for, can do that.

Union bosses typically seek monopoly bargaining authority as a top priority in a collective bargaining agreement, because it maximizes their power. Bargaining on behalf of non-members isn't something forced upon union bosses, but something they actively seek out and seize whenever they have the opportunity.

See:

Federal Law Does Not Force Unions to Represent Nonmembers 

So-called union representation is foisted on workers, not vice versa 

So-Called Burden of Monopoly Bargaining Is Actually Optional