As per CG's: 'If you wouldn't say it in public, don't post it on Answers.' How do they know what I say public? Does Yahoo spy on it's users?

Each of us are different and it's likely likely that both you and I would say very different things in public and have different ideas of what is appropriate or not. So in that respect it's not a statement that can be used in isolation as a rule of thumb.

So that's why the Community Guidelines are there with more details and that statement is more intended as a generalisation.

Whatever rules Yahoo lays down will be open to being pulled apart and critiqued for one reason or another so in that respect they can't win unless they deploy a "anything goes" site and in which case it would revert to chaos in no time at all and close down (same as Yahoo Chat did)

As you know full well, it's a generic "you", referring to general expectations of public conduct.

It's not usual, for instance, to be belligerently offensive in public. That's the standard you'll be held to here, and rules-gaming to argue that you're normally so in public won't wash.

No spying and the "You" in that general expression is a general "You" as in "the user". I always use a more specific rule of thumb. If you would not say it to your grandmother's face, don't say it here. People use very foul language in public they would not say to their grandmother.