From this answer about The Dark Knight Rises:
John Blake to Gordon: Those men locked up for eight years in Blackgate, and denied parole under the Dent Act, based on a lie?
As the answer explains,
The Dent Act. among all things puts dangerous and important cogs of the organised crime world into prison with no possibility of parole. So essentially they are not gonna see the light of the day till a long long time and deter people from being involved in such activities.
Is it bad, to put cogs in organised crime and put such people in prison for 8 years and deter them from such activities? What he complain about?
Perhaps they put them in prison with no real evidence?
If so, why didn't they do that earlier? What allowed them to it and how is Dent related to this?
Are they sure they actually did anything bad?