|The phrase "bad apple" is used by corporate media in cases of blatant corruption. This generally avoids enquiry into the systemic causes of corruption.|
The phrase "bad apple" is a useful explanation of corruption, since its focus is on individual rather than systemic shortcomings.
While even Western democracies may occasionally deceive people, it is unthinkable that this could be a systemic problem. Such occasional occurrences are instead due to a "bad apples", who must be rooted out and removed.
If not removed, a single bad apple can cause a whole barrel to go rotten, hence suggesting the metaphor of the "bad apple" - a corrupt individual within an otherwise untainted organisation.
The metaphor is used by commercially-controlled media when corruption is so blatant that it cannot be denied. Rather than investigate systemic problems, this label is used to focus blame on individuals -- in this sense, it is similar to the "lone nut" meme which is used to prevent deep investigation into deep political assassinations.