In the article title it says for the FBI, but in the article itself it says with the FBI. In this case, I think its just semantics because it was a sting operation so while he was working with them, he was also working for them as well.

To add a point to the article, I think they went too far in their operations saying to "open up on any government operation". Civil Disobedience, and taking down websites is one thing, leaking sensitive information is an entirely different story. This is also Fox news, so I don't know how bias or if this story is even the truth, so I'm taking it with a grain of salt.

Colloquially speaking, "for" means the FBI is making matching contributions to his 401k, "with" means he is cooperating.

Technically speaking, either word works, but you need more than the OED to properly communicate meaning.

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".

I am also an English speaker living (and raised) in the northeastern United States, and I agreed with his differentiation.

I agree with the differentiation, although there's another rule not yet mentioned:

News article headlines are allowed greater latitude to break other grammatical rules.

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.

The article does have a quote using 'for.' '”They caught him and he was secretly arrested and now works for the FBI,” a unnamed source said to be close to Sabu told FoxNews.com.'

Certainly not a very credible citation, but it shows that the poster didn't generate the sensationalist title from thin air.

An NPR story earlier today said he was arrested and pled guilty, agreeing to a plea bargain with the FBI.

I'm too lazy right now to dig up the link on NPR's site though :(