Many liberals are PO'ed. They want more. When you are blogging or chatting with friends it is very easy to state your demands. When you are on the floor of the House of Representatives it is an entirely different matter. You have to get about 250 of your colleagues to agree with you. That doesn't include the Senate where another 50 - 60 colleagues need to agree and then the president.  An immediate pullout just ain't in the cards right now.  So, what's the next best thing?  Place constraints on the spending bill.  That's it.  The Dem's have tried several other avenues.  This one was open.
With the War in Iraq the Republicans will tell you in private what a disaster the war is but on the floor of the House they continue to vote with the president. The president will never admit defeat unless it is in his political interest. With approval rating between 29 - 34%, what does the president have to lose?