"Must be" is the language of a necessary condition. In other words, it is required. "Deceitful" by itself does not guarantee that the job will be done properly, which is why (D ---> JP) is incorrect. It is not a sufficient condition.
Maybe it will make more sense if I rephrase the sentence. "The job will be done properly only if they are deceitful." This has the exact same meaning, and "only if" introduces a necessary condition. JP ---> D not D ---> not JP
