The app could detect security anomalies and whether or not your device was jailbroken.
Here is Apple reasoning for pulling the app...
2.19 - Apps that provide incorrect diagnostic or other inaccurate device data will be rejected
22.2 - Apps that contain false, fraudulent or misleading representations or use names or icons similar to other Apps will be rejected
"We noticed that your app provides potentially inaccurate and misleading diagnostic functionality for iOS devices to the user. Currently, there is no publicly available infrastructure to support iOS diagnostic analysis. Therefore your app may report inaccurate information which could mislead or confuse your users. We encourage you to review your app concept and incorporate different content and features that are in compliance with the App Store Review Guidelines."
Esser says, "they keep ignoring the big question: why kill us and not the other jailbreak detectors or system info tools or iokit using tools..."