Joe's head is the price paid by establishment Democrats in return for the power of the netroots.
The Progressive Blogs aren't going after Landrieu, Pryor, the Nelsons, and many of the other conservative democrats in office. Nor are they standing on the sidelines after a netroots candidate loses the primary to an establishment or incumbent Democrat. The Progressive Blogs are throwing their full collective weight behind every Democrat in every general election against a Republican. Witness the huge fundraising efforts of the netroots behind John Kerry after he defeated the netroots candidate, Howard Dean.
So, I'm sure many establishment Democrats are asking the question... well why wouldn't the netroots back Dem's since Republicans are so bad?
Answer:
1) there was a small grain of truth in Nader's 2000 campaign, i.e., at times, if you squint just right, there seems to be little difference between Democrats and Republicans. What the netroots want are bold, progressive, reform candidates; however, what the establishment Democrats hand back are often candidates who fall woefully short of this ideal. But more importantly...
2) the progressive bloggers had an alternative. They could have thrown the full weight of their power into building a new party. That was the choice: reform the existing Democratic Party, or start a new party.
The progressive bloggers collectively chose the former. They chose to exercise their considerable new power on behalf of a party which time and again, had disappointed them. But they decided this was the more prudent course.
And Joe Lieberman is the sacrificial lamb that the establishment Democrats must yield to the netroots movement. The progressive blogs and the netroots cannot prove their power to the skeptics in the party and the media except by taking down a disloyal incumbent within their own party. It had to happen. Only then, will the skeptics take notice. It's not a case of Herod vindictively demanding the head of John the Baptist. This is a case of 1 example being made, so that the full power of the netroots can be recognized by all and used for the greater benefit of many Democrats, of all stripes.