It appears that Matthews' new hobbyhorse is the apparent hypocrisy of liberals who do not support Libby's commutation and yet also decried the impeachment of Bill Clinton. Matthews' logic here, however, is flawed for the reason that usually gets him into trouble: his knowledge of politics is a lot wider than it is deep. Interestingly, unlike David Broder, who glosses over the details, Matthews frequently loses sight of the forest through the trees. Perhaps he just has to spend so much time listening to spin that it's hard to keep him perspective properly oriented.
Libby's commutation=Clinton's impeachment is wrong for 2 reasons:
1. Bill Clinton was never convicted. Chris Matthews has on multiple occasions brought up this canard about how 50 Republicans voted to impeach him, but it's irrelevant because 50 other senators, plus the VP, voted to acquit (and as we've seen, getting 50 Republican senators to vote for something unjust is not exactly the hardest thing in the world to pull off, especially when they think it will benefit them politically). Civil charges, meanwhile, were not pursued (h/t Media Matters, .pdf).
2. There was no underlying crime. Clinton supporters have been on the rampage about this for years, and they're completely consistent on this point: Clinton lied about cheating on his wife, and adultery is not illegal. Outing an undercover CIA agent is a felony. That Fitzgerald chose not to prosecute Karl Rove and/or Dick Armitage for the leak itself is irrelevant, as prosecutors do sometimes choose not to litigate crimes for a variety of reasons. The act of outing a CIA agent is a felony, and it has been attested again and again, by Fitzgerald himself and by various of Plame's former superiors, that she was undercover and working on Iranian WMD at the time of her outing. Ergo, Scooter Libby lied to cover up a crime, whereas Clinton lied to cover up an embarrassment.
It really takes deliberate obtuseness (or way too much time spent listening to other people's bullshit) not to understand this very simple point.