"FBI wiretap transcripts" sound like very private conversations, probably including innocent people and innocent topics. Just because they had a court order for the wiretap doesn't mean the conversations are now public record. So they are exempt, but then it's still private and not something that you should be taking home.

> prosecutor already had access to them

I don't think that argument works. The same could be said for a software engineer. He had access to the source code, so why can't he keep a copy of it?