Federal prosecutors will offer deals to those with lesser direct offenses in exchange for them to roll on their cohorts and terrorist cell leaders.
Michael Fortier was able to make a deal, and testify against Timothy McVeigh. Fortier got 12 years in prison without the possibility of early parole, and he was extremely grateful for that. He was facing far worse.
These terrorists could be facing 50 to 100 years in prison, or even the death penalty. A generous deal might drop that down to 10 years, or even two or three years. And with decades of monitored parole after that. Anything under a year that allowed them to ever legally own a gun would be a slap in the face of any form of justice. I don't care who they testify against, they have to keep their expectations realistic. The parole monitoring is also essential to keep them from owning guns on the sly.