Another Misleading News Headline

The New York Times online has a story up entitled, "U.S. Forces Mistakenly Kill 6 Afghan Police." If you just read the headline, you would naturally assume that his was a U.S. military screw-up. But if you read the article you find that 

A statement issued jointly by the American and Afghan military commands said a contingent of police officers fired on American forces after the Americans had successfully overrun the hideout, killing the suspected Taliban commander and detaining another man.

So, unlike what the headline implies, U.S. forces had carried out a successful operation when they were mistakenly fired upon by Afghan police. Only then did they return fire, killing them.  Why use a headline that while technically correct, badly distorts what happened?