Why? Because some people may not realize that their posts could be shared externally and may have jobs that could incur impact from sharing something religious or political or even off-colour.
I generally don’t say anything that, when pushed to it, I would back-pedal on but it really is a reputation issue and could really put someone in a bad position. So, while it really is the responsibility of the person posting to not posts things that they don’t want shared, be courteous and keep your amplification of private posts to your own friends and think about whether they would really want it tagged publicly.
And, for those of you posting, keep this post in mind when you put something on Facebook, visible to all your friends. Is it something that you would be upset about if your employer saw it? A potential employer? Then think twice before posting. Just because you marked it friends only doesn’t mean people won’t share it and your name will still be attached.