I know that's what the NWU says, but it's wrong. There's a substantial legal barrier: antitrust law... because truly freelance writers aren't eligible to unionize. The NWU has essentially been grandfathered in, because so much of its membership produces WFH (work for hire), particularly in periodicals and in work that is assembled from periodicals. It's rather ironic that WFH authors can organize, but freelancers — who need those protections even more than do "employees" producing WFH — cannot do so (or at least cannot do so legally).
Freelance writers are not employees. They are management; they manage independent contractors. That means that any collective action by freelance writers that might affect price or availability of their goods falls inside the antitrust laws, but outside the union exception to the antitrust laws. Under US law, only nonpolicymaking employees can unionize, and the owner of a business by definition is policymaking.
That said, there have been a couple of attempts, most recently HR 4346 of a few years ago, to explicitly give artists, authors, etc. the right to bargain collectively. Both Hollywood and the music industry successfully bottled those attempts up in committee; the publishing industry was able to remain aloof because its allies did the work for it. (Hollywood's fear concerns graphic artist, not writers — the writers are already unionized, because they're producing WFH.)
