Probably because they were accepted into the store before they tightened the regulations. Have a read of this thread on appmakrs own forum:
e.g an appmakr user that was denied received this from apple:
"We’ve completed the review of your app, however, we cannot post this version to the App Store because it is only a content aggregator and provides a limited set of features and functionality to users. Such apps do not comply with the App Store Review Guidelines <https://developer.apple.com/appstore/resources/approval/guidelines.html>:
12.3 Apps that are simply web clippings, content aggregators, or a collection of links, may be rejected"