Shadow evidence sufficient to oppress
"we have secret evidence that proves his guilt, trust us" doesn't cut it. Perhaps with the opportunity to review and contest the evidence, an innocent man could be spared from unjust punishment,
The Government knows full well that "secret evidence" that will allow a Federal Prosecutor to file charges is sufficient to achieve their aims, simply by destroying the defendant through catastrophic legal fees, destruction of reputation, probable loss of employment, and other personal damage such as impact on family relations and possible divorce proceedings. More significantly, the wide circle of fear and confusion that spreads through the community: "don't risk government displeasure" is significant. The FBI is indeed spying on pedophiles and terrorists, but is equally concerned with internal dissidents and domestic "opposing forces," whoever and whatever they may be. It established itself as an unapologetic watchdog during the Civil Rights and Vietnam War protest eras, and has simply gone "quiet" while ramping up internal surveillance abilities.
Also, please remember that "public defenders" in the US are underpaid, overworked, and inexperienced. Public defenders are regarded as a burden on the system, employed at great taxpayer expense to defend the undeserving. Typically, public defender offices are ill-funded and under-manned in most jurisdictions. Yes, justice is available... to those Americans who can afford it. Those without means may get a plea bargain prior to sentencing.