Apple doesn't apply this to it's own content on iTunes?
It's not Apple's content. The content belongs to others, and Apple has no right to alter it. You can suggest that audiobooks have an "explicit" tag on them similar to those on music tracks:
iTunes feedback page
but that would require the cooperation of the owners of the audiobooks, so it's not something that Apple can implement quickly, if they can at all.
Don't you find it interesting that the Apple forum restricts language ( the asterisks in my posting were automatically inserted by the Apple forum) but Apple doesn't apply this to it's own content on iTunes?
Not really, no. The forum software has a search engine built in that can look for certain words. That's easy to accomplish. It's impossible to automatically scan an audio track for explicit language and filter it automatically. As I said, the owners of the content would have to cooperate in labeling their material so that Apple could provide a filter as they do with music.
they likely don't fully understand the Public Relations issues at play here. The target kids with iPods and also access to iTunes.
Of course they do. But they're limited, as is any content provider (particularly one that doesn't own the content), in what they can in practicality do.