I think DRan has it correct. The big agencies in DC have contracts with shredding companies (maybe still Iron Mountain?). I was at the Pentagon for 5 years. Shredder trucks showed up once or twice a week (scheduled on the same days) at the east loading dock - no idea how many others went to other loading docks. My co-worker and I still both smoked and we would go out there for either a break or to clear our heads if we were hitting a brick wall in one of our penetration tests.

The trucks would be there for a couple of hours. Most weeks we would see them twice, but every once in a while they wouldn't be there on one of the days, so I bet they were called and told they weren't needed this time.

Now I'm sure more stuff is digital these days (as opposed to the late '90s) but the Pentagon went through a LOT of paper. I'd wager they still do. This doesn't mean the DOJ isn't trying to get rid of bad (to them) info, but the DOJ I'm sure has a ton of paper files/docs for use in court that would need to be shredded once used.