Commuting Stone’s sentence allows Stone to plead the Fifth if called to testify against Trump.
It’s case law that was used when President Bush commuted Scooter Libby’s testimony. (tagging @Shewmaker)
“President Bush's decision to commute Scooter Libby's may seem like some kind of compromise between a full pardon and sending Libby to jail.
“It may be more sinister than that. Since Libby's conviction stands, Libby will almost certainly continue his appeal to have his conviction overturned. His many Republican supporters will probably defray the costs of the appeal so it will cost Libby nothing.
“Now, with an ongoing criminal appeal, if Libby were called before a Congressional Committee and asked to testify about efforts by the Vice President's and President's offices--including Cheney, Bush and Rove personally--to discredit Joe Wilson and reveal that his wife Valerie Plame worked for the CIA--“Libby can refuse to testify by taking the Fifth Amendment against self-incrimination. If Bush gave Libby a full criminal pardon, the Fifth Amendment would no longer be applicable and Libby could probably be forced to testify or face jail for contempt of Congress.”
And why doesn’t Trump want Stone to testify?
Unredacted portions of the Mueller report show that Paul Manafort, Rick Gates, and Michael Cohen told investigators Stone had promised the campaign damaging revelations by WikiLeaks.
The information was released in June following a lawsuit by BuzzFeed News:
“In July 2016, political consultant Roger Stone told Trump as well as several campaign advisers that he had spoken with Julian Assange and that WikiLeaks would be publishing the documents in a matter of days. Stone told the then-candidate via speakerphone that he "did not know what the content of the materials was," according to the newly unveiled portions of the report, and Trump responded "oh good, alright" upon hearing the news. WikiLeaks published a trove of some 20,000 emails Russians hacked from the Democratic National Committee on July 22 of that year.
“The new revelations are the strongest indication to date that Trump and his closest advisers were aware of outside efforts to hurt Clinton’s electoral chances, and that Stone played a direct role in communicating that situation to the Trump campaign. Trump has publicly denied being aware of any information being relayed between WikiLeaks and his advisers.”