There were good reasons for the DNC to add the states it added to the January primary calendar. In addition to the mostly white populations of Iowa and New Hampshire, labor unions and Hispanics in Nevada and blacks in South Carolina were included among the voters in the first four primaries. Including these states added to the diversity of the process.
The DNC also made a good move in excluding large states in the early contests. Large states are particularly expensive to compete in, and too many of them too early would turn the primary into a fundraising contest. As it was, comparatively underfunded candidates like John Edwards were able to seriously compete in early states and shape the debate.
Most importantly, keeping any other states from jumping in line was essential to keep the primary calendar from collapsing. What was to stop Vermont from deciding in summer 2007 to move their primary to December, and California from one-upping them into November, and New Mexico from giving their governor a boost by holding a surprise primary in mid-October? That kind of chaos wouldn't allow voters in the early states to seriously consider their vote.
So there were good reasons for the DNC to pick that particular basket of states early, and further good reasons to make sure their rules were followed. When Iowa and New Hampshire moved their primaries slightly earlier, none of the principles underlying the DNC's decision to schedule the primary calendar as they did (diversity, small states first, preventing surprise primaries in October) were violated. So the DNC didn't do anything.
But when Michigan and Florida jumped into January, the DNC had to enforce their rules or risk seeing the primary calendar collapse. So they penalized these states, and none of the candidates protested. As bad as it is for people in Michigan and Florida to not have their votes counted, this is the kind of punishment that has to be built into the process for it to make any sense. Without these constraints, there's no telling what kind of crazy things state party bosses might do with their primaries. The DNC has to act to preserve the integrity of the process.
Lots of people have written about how it's ridiculous for Hillary Clinton to accept the DNC's decision back in January, and only complain about them now that it's to her advantage. My point here isn't that -- if anything, it's a more controversial one. Howard Dean and the DNC made the right decision back then, and they'll be right to enforce their rules now.