Eliezer,

With delegates divided in accordance with the vote, South Carolina delegate slots for the Democratic convention are a scarce resource to represent the substantive preferences of the party. If Colbert were to somehow exceed the 15% minimum threshold, other state parties, possibly with different priorities, would predominate. Worse, from the state party's perspective, Colbert could encourage people who would otherwise not vote in the primaries to participate as a joke.

Primaries are not supposed to best express the popular will, but are creations of political organizations to select candidates who will advance certain policies. You could compare this to the Google founders retaining stock with multiple-voting rights in their company, with full disclosure, when selling equity to other parties.