Antitrust has a sort of inherent tension with IP. One stops monopolies and the other grants monopolies.
The core principle is that it is OK to earn supracompetitive profits as "a consequence of a superior product, business acumen, or historic accident". It becomes a problem when there are no adequate substitutes, or alternatively, you take steps to ensure that there are no adequate substitutes.
The concept of "substitute" applied to HP essentially says, JK Rowling wants to maximize her sales. If she signs an exclusive license with Scholastic, we can trust that she is bargaining for Scholastic to do as good a job as possible publishing her works. Likewise, we can trust that Scholastic also wants to do as good a job as possible. Sure, no matter how crappy their product is, they'll still have some consumers, but when they make a good product, they'll attract more consumers that are currently buying substitutes (Eragon?) instead. So if Scholastic dramatically overprices HP, it is a net loss for them.
On the flip side, if there were no substitutes (let's say Rowling's contract with Scholastic also required it to stop publishing other teen fiction novels, and Scholastic dominates the teen fiction market), now the incentives are all wrong. Now there is a lot less reason to worry about overpricing: your consumers are captive because they don't have anywhere else to go. If Scholastic produces a superb product, they don't gain many more consumers than if they produced a shabby product.
Now, of course, there are gradations. Antitrust is nothing if not murky, and sometimes the ultimate verdict kind of comes down to a gutcheck.
The disconnect might be how you define the relevant market. The history of US antitrust works very hard to promote "interbrand" competition, rather than "intrabrand" competition. Reason being, if you define each individual brand/product as its own relevant market (i.e., Harry Potter's market is Harry Potter books, not all books), then you run into some other problematic issues. Is it OK if Pepsi agrees to sell only west of the Mississippi and Coke agrees only to sell east of the Mississippi? Shouldn't it be an antitrust violation if DXV pays RGG/etc. not to make any other deckbuilders? Neither of those would be prosecutable if each brand constituted its own relevant market, even though there might be compelling reasons to suggest that Coke and Pepsi are not "true" competitors, just like how Dominion and Ascension are not "true" competitors.