Sales of digital comics have soared in the past three years. Readers love the look of comics on the iPad screen and they also love the convenience of in-app purchasing, which allows consumers to buy and store their comics within a single app. So it’s a big deal when Apple bans a comic—usually because of sexual or mature material or nudity—and it has happened to at least 59 comics this year.

The problem, publishers say, is that it is unclear what criteria Apple uses to judge whether a title should be banned. The most recent case of banned comics involves writer Matt Fraction and artist Chip Zdarsky's series Sex Criminals, published by Image Comics. Sex Criminals is a comedy about a couple who can stop time when they have sex—and use that power to rob a bank. Apple approved the first issue, published in September, for in-app purchase. The second issue was rejected. Last week, Image Comics was notified that the third issue had also been rejected—and now the first issue has been retroactively banned as well. In an e-mail interview, Image publisher Eric Stephenson said that no reason was given for the rejection. "We received notification from Comixology, not directly from Apple, but they give us no explanation whatsoever," he said. "They just say they’ve rejected it."

The digital comics marketplace is served by such vendors as Comixology—easily the biggest and most popular comics app—as well as apps by Comics Plus and others. When Apple rejects a comic it means readers can't buy them from within an iOS app, although they can still buy the comics on the Comixology or Comics Plus Web storefront or Android platform and sync them on to their iOS devices from there. Banned comics also are available in the Apple iBookstore, but the iBookstore doesn’t sync with Comixology or other comics apps. In other words, being kicked over to the iBookstore doesn’t help discoverability.

A Comixology spokesman declined to comment for this article, and Apple did not respond to requests for comment.

Although readers can buy Sex Criminals on other platforms, including the Image Web site, that doesn't compensate for the loss of in-app purchases, Stephenson said. "It absolutely hurt digital sales on Sex Criminals #2," he said. "This is a series that is getting fantastic word of mouth, it’s amazing work by Matt and Chip that is receiving rave reviews and selling out instantly. Not being able to offer the book to curious readers through our app or the Comixology app is a significant deterrent to reaching the widest possible audience."

"The Comixology app and the Image Comics app that runs through Comixology are far and away the most important of all the digital platforms we’re working with right now," he said. "the iBookstore is a distant second."

For his part, Stephenson doesn't believe Apple's standards are clear. "Their standards are kind of vague, really," he said. "I get the impression that either not a lot of thought went into how they were handling mature content, or worse, there’s an agenda to position Apple as some form of moral arbiter for things like comics. And yeah, I absolutely think they should change the standards, because I don’t think there’s anything particularly offensive or obscene about a book like Sex Criminals."

Apple's guidelines have proved confusing in other cases: In April, popular comics and TV writer Brian K. Vaughan said that Apple had banned issue #12 of his comic Saga (also from Image) from in-app purchase. A few days later, Comixology clarified that the decision had been theirs, explaining that, "Based on our understanding of those policies, we believed that Saga #12 could not be made available in our app, and so we did not release it today." As it turns out, Saga #12 was approved by Apple.

After that, Image requested that Apple review three of its titles, Howard Chaykin's Black Kiss II, Joe Casey and Piotr Kowalski's Sex, and Rick Remender and Keiron Dwyer's XXXombies. "We requested the review because we felt those books should be available through our apps, which remains the case," Stephenson said. Initially, Apple approved all three.

However, in May, Apple notified Comixology that 56 comics that had previously been available on its app did not meet their standards and must be removed. That list included the three recently reviewed Image comics as well as issues 1-13 of Johnny Ryan's Angry Youth Comix and books 1-4 of his Prison Pit, Hans Rickheit's Folly: The Consequences of Indiscretion, and the LGBTQ anthology No Straight Lines (also nominated for an Eisner Award), all from Fantagraphics Books; five yaoi manga (romances between two males) from Digital Manga Publishing; Jess Fink's Chester 5000, published by Top Shelf; Chaykin's Black Kiss and Grant Morrison's The Boys: Herogasm, both from Dynamite Entertainment; Omaha the Cat Dancer and The Story of O, from NBM; and a quintet of titles, including Chicken Soup for Satan, from the horror/fantasy publisher Asylum Press.

Being excluded from the Comixology app is serious business for publishers. The digital distributor was the third highest grossing iPad app in 2012, and in September they reported that comics downloads had reached the 200 million mark.

Stephenson said when Image comics are banned from in-app purchase, "we make sure people are aware of what’s going on and all the various alternatives. There are people who view all of this as some kind of lavish publicity stunt, but you know, how else are we supposed to respond? The whole point of publishing this stuff is to get in front of as many people as possible. If we’re notified we can’t sell through one channel, of course we’re going to direct people to the others."

Asked whether he thinks editors and creators are keeping Apple's standards in mind as they create or edit a comic, Stephenson responded, "I guess they might, but we certainly aren’t telling people to change anything. I mean, we’re doing the Black Kiss II Christmas Special, and I guarantee you Howard Chaykin isn’t toning anything down to suit Apple’s prudish tastes."

As for whether the standards will affect Image's digital strategy going forward, he said, "Not immediately, but it’s certainly an incentive to broaden our efforts."