Setting aside whether Scooter Libby should spend 0 days in jail for what most people spend from 1 to 3 years in jail, the key here is that it's inappropriate for the president to pardon or commute a sentence in a case in which he (i.e., the president) is a party to the same underlying crime. Because it amounts to obstruction of justice.
Note how quickly the tables have turned here. People (like Marshall) who bemoaned the guilty-until-proven-innocent attitude of Republicans during the Clinton years have now decided -- based on no hard evidence -- (a) there was an underlying crime and (b) that President Bush "is a party to" it. To believe this to be true, you have to believe that Richard Armitage innocently leaked Valerie Plame's status to Robert Novak before other Bush officials could unleash a plot that demonstrably violated the relevant statute. In addition, you have to believe that Libby's testimony would reveal this plot. While it's possible that all of this happened, assuming that it did is completely unreasonable.
Matthew Yglesias is more circumspect about the existence of an underlying crime, but still frames the issue in a "when did you stop beating your wife?" style:
Most of Libby's defenders -- George W. Bush, David Brooks, etc. -- don't seem to be denying that Libby committed a crime by lying under oath to investigators. They want us to say that, rather, he deserves to be treated very leniently because there was no big deal here. The alleged absence of an underlying crime is key to that theory. The converse theory is that there was an underlying crime and the crime can't be proven because Libby lied to investigators.
If that theory is wrong -- if there really was no crime -- then it seems we ought to get some kind of explanation from Libby as to why he lied. People sometimes do have reasons to lie to investigators other than a desire to cover up criminal activity (hiding non-criminal activity that's embarrassing is the obvious one) but if Libby wants mercy he should offer up a plausible score on this account. But Libby hasn't offered any such story... Maybe there was no crime here; but if there wasn't, then what was Libby doing? He's not even trying to convince us that he had some other reason to lie.
Libby is by all accounts a loyal servant. Couldn't he just be protecting his superiors from exposure of embarrassing but non-criminal conduct? In the end, we have no idea what happened. There is no proof of a criminal conspiracy. Asserting or speculating that one took place is irresponsible.
Update 7/6 10:37 AM: Yglesias concedes a great deal of ground in a response, changing the question from "Doesn't the absence of an explanation for Libby's conduct suggest the existence of an underlying crime?" (the subject of his previous post) to "Was Bush's decision to commute Libby's sentence an abuse of power?":
Brendan Nyhan criticizes liberals for simply assuming the existence of an underlying crime in the Scooter Libby case and sweeps my post here into that rubric, wondering of Libby "Couldn't he just be protecting his superiors from exposure of embarrassing but non-criminal conduct?" He certainly could. But let's assume that's true. Is "I broke the law to help my boss cover-up embarrassing but non-criminal conduct" a reasonable case for lenience? No. Is "he broke the law to help me cover-up embarrassing by non-criminal conduct" a reasonable case for granting someone clemency? Also no.
The bottom line is that on one theory -- Libby broke the law to spare his superiors embarrassing revelations of their lawbreaking, and is being pardoned by those same superiors to help perpetuate the cover-up of their embarrassing lawbreaking -- Libby deserves to go to jail and Bush has seriously abused his power by pardoning Libby. On Nyhan's alternative theory -- Libby broke the law to spare his superiors embarrassing revelations of their embarrassing non-criminal conduct, and is being pardoned by those same superiors to help perpetuate the cover-up of their embarrassing non-criminal conduct -- Libby also deserves to go to [jail] and Bush has also seriously abused his power by pardoning Libby.
This -- that the President of the United States is abusing his power in a serious way -- is a substantially more important issue than the question of whether Josh Marshall should be slightly more circumspect in his characterization of the serious abuses of power.
Actually, unsubstantiated assertions that the president of the United States participated in a criminal conspiracy are a pretty important issue. (Matt's liberal friends were pretty worked up about this issue a few years ago -- and rightly so.) In any case, despite my misgivings about the Libby case, I ultimately agree with Yglesias that Bush's decision was an abuse of power and I support the proposal he highlighted that would amend the Constitution to ban pardons and commutations for "executive-branch employees convicted of crimes carried out in the course of their professional duties."
Update 7/6 12:41 PM: Yglesias added the following update to his post:
UPDATE: See Brendan's update. Bottom-line, I think it's rock solid that Bush abused his power, and until someone can offer a plausible account of what kind of non-criminal conduct Libby is helping to cover-up, I'm not going to be too upset if people assume that what's being covered-up was, in fact, a crime. The fact that Bush is actively and openly participating in the cover-up (and there's no serious doubt that something is being covered-up) naturally whets one's suspicions. Bush and Cheney are, however, clearly entitled to a legal presumption of innocence.
I'll just add that many conservatives had many such "suspicions" about the reasons for alleged coverups of, say, Whitewater that turned out to be unfounded.
Update 7/7 10:32 AM: Conversely, as Media Matters points out, Robert Novak is incorrect to assert definitively that "there was no underlying crime." As a result of Libby's perjury and obstruction of justice, we don't know that for sure either.