This rule was implemented by a
community vote that closed on June 15, 2012 in which the voters approved it by way of voting down the permanent adoption of a
prior approved 3-month temporary ban on personal attacks and insults in P&N.
Basically the community initially wanted the mods to enforce a no-insults policy, and we did for 3 months. Then the community decided they didn't want the policy anymore, so they voted it down.
Like any citizen-derived
initiative, if and when enough members of P&N come to the mods (as you have done here) requesting a re-vote on the no-insults rule then I will open yet another mod-sponsored community poll on the matter.
At this time there has not been enough interest expressed from the community to put this issue "back on the ballet" so to speak.
So, for the indefinite future, direct insults are allowed in P&N until such time that the community tires of it and wishes to do something about it (again).
If you feel strongly about the matter then feel free to begin your own grass-roots campaign to enlist enough petition-signers to see the "no insults" rule added back to the P&N guidelines.