As the quotes from the editorial below note, by commuting Libby's sentence instead of pardoning him, Bush has created a perfect scenario for Libby - the possibility of a pardon being the carrot to keep him quiet and leaving the conviction intact so that he can continue his appeals, which means he can avoid testifying anywhere by claiming his 5th amendment rights. Had Bush outright pardoned him, Libby wouldn't face the possibility of his words potentially incriminating himself since he'd already been absolved of any criminal liability, and thus could be compelled to testify in further investigations - such as an investigation into Cheney's involvement in the CIA leak scandal.

So, while the President tries to claim that he commuted Libby's sentence as a means of correcting what he thought was an injust sentence, the President has actually managed to thwart attempts to bring others involved in case to justice.