Because breaking from one's party is only valued by the left if that person is a Republican... it amazes me that in today's politics where we look at the Tea Party and other extremists with disdain and we criticize people such as Romney for pandering to his party base- even in this world we brand some one like Lieberman with the label of traitor because he dares go against his party. Here is news for you- Lieberman is a liberal with the exception of a few issues... on this issues even if you disagree with him, you ought to admit he took an extremely courageous stand based on his moral convictions...something that is all too rare in today's politics
Your understanding of why liberals find Lieberman vile is all wrong. Lieberman, a man of convictions? Lieberman is a man of convictions when they line up with the convictions of insurance companies, the military industrial complex, status quo nanny-statist, etc., and when they don’t, as we saw with the public option in the health care debate, he changes those convictions to match those to whom he serves.
Same argument could be made for any politician... change positions for campaign cash or to match public opinion polls