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It seems that Apple has intentionally disabled the jailbreak detection API in its iOS. As to why the company has done this, no reason has been revealed so far. Well, just a couple of months ago, Apple has said that jailbreaking is legal, although the process may still void the warranty of the iPhone. 

All iPhone owners need to use the company's proprietary iTunes software to install and manage their apps, which can pose to be rather restrictive, unless one choose to jailbreak their mobile device. A jailbroken iPhone lets you use other alternatives to load and modify apps outside of Apple's iTunes app store, although it is still susceptible to vulnerabilities which could exist on the iOS version that the iPhone is equiped with. Thus, the company initially introduced the jailbreak detection API into its iOS to ensure non-jail broken phone are secured, especially for enterprise users whereby jailbroken phones can pose huge security threats.

Source: PC World