Doubtless because your letters improve the quality of the writing and average IQ of the paper's content. At no cost to themselves.
That said, good job Fisking Ms. Slaughter and her "sources". Everybody in the White House Press Corps sat there and nodded their heads on every one of her points. The MSM can't be bothered to research their stories anymore. Might counter their bias...
There's more to it than that. The DN is a pretty scrappy, feisty outfit. And they always tend to keep a token attitudinal conservative on their editorial board. Sadly, one was killed in a robbery attempt a few years ago - a man named Russ Beyers, who defended his wife and took a knife for it.