Why does any newspaper allow ad hominem attacks in letters?

To The Daily Sun,

I am a reader of other newspapers as well and found the following opinion policy in one New Hampshire paper. And I wonder why any other newspaper would not have such a policy that precludes ad hominem letters:

"Letters to the editor should be 250 words or less in length, and “Your Turn” guest column between 500-800 words. No unsigned or anonymous opinions will be published. We require that the person submitting the opinion also include his or her town of residence and home telephone number. We must authenticate authorship prior to publication. We reserve the right to edit or withhold any submissions deemed to be libelous, unsubstantiated allegations, personal attacks, or defamation of character.

Len Campbell

Center Harbor