How would they prove that they have changed from their bad habits if they aren't allowed to post? Does this set of users receive a prompt message to contact the administrator for a second chance? It is typical for anyone to abandon their profile if their ability to post is taken away.
Parma/Temp ban users get a prompt message on their profile on BTT.
I seem to remember that even being in that group (Under Review), users can send PMs to other users, but even if I'm wrong (which I could be), the user can always email the administrator or create a new temporary account to find out what happened. Communication options are always available when someone wants to talk and ask for another chance. When a user belonging to that group really wants to change their behavior, they have many ways to talk to the administrator or any moderator. Not being able to post openly on the forum I think is a very simple excuse.
Only the admin can't understand the behavior of people. The user has to come to the public to display his or her character to everyone that he is has changed and that will be how the person response to comments and the thread is creating. The character of the person would display to the admin might not be the same with other people. Private discussions is good but the person will still come to the public to interact with others. So it is good for the person to come to the public. And the person must have a thick skin because definitely there must be an attacks.
Change is a constant thing and it must be in gradual process. You don't rush someone to change his or her character immediately but allow them to adapt to the environment.