Apple has started to enforce an App Store rule that requires all screenshots on the store to comply to the 4+ age rating standard, and is removing apps that depict violence or feature guns in their screenshots. According to the App Store guidelines document:
The App Store has parental controls and requires all apps to bear age-appropriate content ratings. While violence, etc. has always been permitted in apps, Apple has always required that all app metadata — title, description, icon, and screenshots — be kid-proof with the lowest rating. [...] It's right there in the rules: 3.6 Apps with App icons, screenshots, and previews that do not adhere to the 4+ age rating will be rejected.
Pocket Gamer noticed that Tempo (image above) featured a blurred gun in its App Store screenshots to adhere to the ruling. However, other action games such as Modern Combat 5: Blackout still have guns highlighted in their screenshots:
Gunslugs 2 developer OrangePixel also found that an update to the game was rejected as the screenshot for the game depicts "violence against a human being." The issue has since been fixed, and the game is now live with the update (and the same screenshot).