Don't call Diamond Comic Distributors a monopoly or they'll get angry with you. I know of what I speak. The charge is usually based on their exclusive rights to distribute Marvel, DC, Image, Dark Horse, IDW and many more comic book publishers in the direct market to comic stores. And it's a situation that resulted from the distributor wars of the nineties.
It's a description Diamond disputes. But their Wikipedia page says that it "has a distribution monopoly structured by exclusive arrangements with most major US comic book publishers"
It's twenty years since an investigation by the U.S. Justice department for possible antitrust violations was started, closed in November 2000, on the basis that, although Diamond enjoyed a monopoly in the North American comic book direct market distribution, they did not enjoy a monopoly on book distribution.
But it is still a touchy subject for Diamond. Which is why it was surprising to find such a charge made by Spongebob Squarepants in the 2017 Free Comic Book Day title Spongebob Squarepants Freestyle Funnies 2017, a title which will have been pored over by Diamond and Free Comic Book Day committee members made up of retailers looking for anything that could cause them problems when handed free to a kid. And hey, it;s not the most uplifting critique of Free Comic B0ok Day either!
Here's the full first page in question, by Jay Lender and Jacob Chabot.