Zazi's lawyer did not have a deal for his client when he first brought him in for questioning and waived his right to remain silent. He assumed his client, whom he had met the day before when he walked into his office looking for another lawyer in the suite who was in court, was telling the truth when he said he had nothing to do with terrorism. During that first day of questioning, it became apparent to the Zazi's lawyer that Zazi was lying. Instead of breaking off communications, he went for a deal.

Folsom and Zazi arrived back at the federal building the following morning with a proposition: Let’s make a deal. “We fully expected he’d be pleading guilty, so we were thinking about sentencing considerations,” Folsom says. The only way Zazi could avoid going to prison would be by providing enough information about Al Qaeda’s operations to get some kind of witness protection. His concern for his wife, children, and extended family in Pakistan—they were well within the reach of Al Qaeda and the feared Pakistani intelligence service, ISI—gave Zazi even more reason to cooperate.

He got a "proffer letter" (not immunity) from the Government:

The government had offered a so-called “proffer letter,” meaning nothing Zazi said could be used directly against him, although the FBI could try to independently confirm his information.

In other words, the information Zazi provided could be used against him not only as impeachment if he later went south on the Government and testified differently, but so long as the Government was able to confirm what he told them through other sources. There's no mention in the article of what benefit Zazi would get for his cooperation if he stayed with it, and I doubt the Government promised any specific result.

As to how the Government got on to Zazi to begin with: from a monitored e-mail.

Zazi had bought most of the over-the-counter ingredients for the bomb at local beauty supply stores—which FBI agents confirmed by examining the stores’ security camera footage—and he’d cooked the first batches of the explosive at Aurora’s Homestead Studio Suites on August 27 and 28. But when he’d tried to ignite some of it in the parking lot, it just flashed and burned.

Zazi then emailed his Pakistani contact for help with his recipe. It was that coded message that the government intercepted, tipping them off to Zazi’s identity. He returned to the hotel on September 6 and 7 and fiddled more with his recipe. He took some of his reworked mixture to a lonely end of East Smoky Hill Road and ignited it—this time, it exploded. Zazi had his bomb. The next morning, the FBI began following him.

...If not for his chemistry failure, the FBI likely wouldn’t have known Zazi’s name until after the New York City attack.

Zazi's attorney had no prior federal court or terrorism related case experience. The FBI felt lucky.

The bureau, of course, welcomed such a weak adversary. “He’s the MVP of this case,” says one federal official with a laugh, “no question.”

Only after the second day of voluntary questioning, which Zazi's lawyer figured out was being secretly videotaped and leaked to the media, and after Zazi told him he wouldn't name names, did his lawyer call of the questioning.

When Folsom and Zazi talked that morning by phone, Zazi told his lawyer he wouldn’t violate his religious beliefs and inform on his friends. “I’d rather spend life in prison than eternity in hell,” he said. Resigned to his client’s impending arrest, Folsom advised Zazi to gather his family and spend his last remaining hours of freedom with them. Make sure to touch them and hug them, Folsom said, because you may never be allowed physical contact with family members again. He left Zazi with one last thought: “When the knock on the door comes—don’t resist.”

If Zazi's lawyer hadn't agreed to questioning that first day, would the feds have been able to build a case against him? Probably, it just would have taken longer. They would have arrested Zazi for some offense, to keep him off the streets while they figured out what he was really up to.

Zazi has done pretty well for himself, in comparison to others similarly charged. He got out of MCC, isn't at Supermax and likely will get some kind of sentencing break. The next we see of him may be at his the trial of Adis Medunjanin in Brooklyn, New York, now set for January.

Justice Department officials say he might still appear at co-conspirator Adis Medunjanin’s trial, set for January. [Co-defendant]vAhmedzay was indicted in 2010 on the same charges that Zazi admitted to earlier, which seems to indicate that Zazi helped law enforcement build its case against Ahmedzay.

Ahmedzay has pleaded guilty and is awaiting sentencing.

Zazi's first lawyer is pleased with the result:

Art Folsom says that a reduced sentence and witness protection for his family was Zazi’s goal from the beginning. “At the start, we wanted witness protection,” Folsom says. “I guess he got there eventually.”

.... All of which means that Najibullah Zazi—the central figure in one of the most dangerous terror plots to target the United States in the last decade—could someday end up free under the guard of the witness protection program, a reward for years of cooperation that began with his arrival at the Byron G. Rogers Federal Building on a sunny September morning in 2009.

In all, a good read.