You also have to read an article to understand its contents. Headlines have never been, and should not be, anything more than a headline.
And, I disagree with your analysis of "for." I am an english speaker living in the northeastern United States, and hear "for" differently than you describe.
In this context, "working for" usually indicates that the person in question is under the FBI's payroll.
"Working with" usually indicates that they're a witness of some sort helping with the investigation. You wouldn't say that a murder witness is "working for the police", you would say that they were "working with the police".
Maybe so if they do it to shorten the headline, but not if it perverts the meaning. I'd say it's okay as long as they get the point across, but I think this is a mistake--it changes the meaning of the title and still doesn't offer much benefit (one character) in terms of brevity.
And, I disagree with your analysis of "for." I am an english speaker living in the northeastern United States, and hear "for" differently than you describe.