It's not a punishment, it's a form of pre-trial custody under restrictions (can't read news, can't have unmonitored communication with people outside) so you can't interfere with the criminal investigation.
I don't see how adjusting his testimony at trial interferes with the criminal investigation. Especially since they kept him in solitary for months, only to release him now, and still no charges are laid against him (just nebulous 'suspicions').
