A screen shot of a search term on Vine that quickly displays adult content. Vine Twitter's new app for the iPhone and the iPod Touch became a haven for porn videos as soon as it was launched last week, and that has raised questions about Apple's policy on allowing apps that show adult content. Pornographic content quickly became prevalent throughout Vine, the Twitter-owned app that lets users upload six-second videos shortly after becoming available for the Apple devices last Thursday. Despite the videos, the app has been allowed to remain in the Apple App Store. Critics are accusing the Cupertino, Calif., tech giant of having a double standard, since it has banned other apps with adult content. Twitter apologized for the accident and explained its policy on adult content, saying those types of videos violate the app's guidelines and are removed. Twitter also said users who post porn can also be banned from using the app. "Videos that have been reported as inappropriate have a warning message that a viewer must click through before viewing the video," a Twitter spokeswoman told The Times. Despite Twitter's move, Apple is separately coming under fire for its app policy. Apple allows apps from such big-name social platforms as Twitter, Vine, Tumblr and Pinterest to remain in its App Store even though they contain adult content. Yet it has been knocking off other lesser-known sites from its store because of "pornographic images and material." Most recently, an app called 500px suffered that fate. Critics say Apple should have a clearer policy that is enforced uniformly.