The most obvious problem is simply that understanding of what copyright does, and does not, protect leaves a great deal to be desired. In this context, we're dealing with both derivative rights and some sense of moral rights. On the one hand, the thread has been used as yet another excuse for some people to rant against what they perceive as excessive copyright terms. On the other hand, the thread simply isn't delving into what makes something a "copy"—instead, I think it (largely rightly) assumes that's a case-by-case determination that is usually too difficult for fanfic in the first place.
The second problem is that, aside from a few nerds (like me), nobody is really looking outside of their own perceptions of copyright law to get any guidance on the issue. Let's look just at the particular instance that inspired the thread for a moment. Does an author's vanity/self-published Star Wars fanfic have any noncopyright implications if sold via the Big Brazilian River? I should think this fairly obvious. For one thing, the Star Wars mark appeared all over both the writer's pages for the book and the Big Brazilian River page (which now gives a 404 error). For another, and perhaps more subtly, the book's description is inherently dilutive (and probably disparaging). On the cached version of the author's website's page, one finds this:
It is a dark time; the evil Galactic Empire rules with a titanium fist. A small band of rebels, led by Imperial Senator Princess Leia Organa, has stolen plans to the Empire's weapon of ultimate destruction, the Death Star, in hopes of finding a weakness that might allow them to neutralize the Empire's fearsome weapon. Then the Empire's most dreaded agent, Dark Lord of the Sith Darth Vader, captures the senator and brings her aboard the Death Star itself for interrogation.
Little does the senator know that she has a secret ally aboard the Death Star, who will help her bring the rebellion into a final, catastrophic encounter with the Empire. And little do the Empire's leaders know that the Dark Lord himself harbors a secret with explosive consequences for the galaxy…
Leaving aside the poor writing (titanium fist?), note the revisionism concerning a specific set of events forming the core of the Star Wars mark.
Third, and perhaps most difficult to explain, the thread never really grasps the problems cause by overloaded meanings of "commercial." To many people, "commercial" is the opposite of "nonprofit," and has come to mean "done primarily with profit in mind." That, however, has little (and perhaps nothing) to do with how the law in general (and, in particular, in the US) treats the term and its foreign cognates. "Commerce" concerns the flow of goods and services between actors; any exchange—even merely the hope of good will—suffices. Only a pure gift is outside the scope of "commerce," and it's becoming increasingly difficult to characterize anything as "purely" a gift. Perhaps the most obvious (and least acknowledged) source of this definition is Supreme Court jurisprudence on the Commerce Clause. In Heart of Atlanta Motel, 379 U.S. 241 (1964), an inn that might conceivably rent rooms to travellers from another state was found to be engaged in "interstate commerce," allowing the federal government to enforce its disapproval of racial discrimination in accommodations. Conversely, cases since the 1960s, such as U.S. v. Lopez, 514 U.S. 549 (1995), that have found an activity outside of "interstate commerce" have foundered not on "commerce," but on "interstate." Thus, we need to pay attention to the stream of commerce—not the profit motive. That, however, is definitely contrary to the Information-Wants-to-Be-Free credo.