I'm pretty sure the official reason for running marathons on holidays is to spare us fans from missing an episode of whatever it is we're used to watching.
They assume that we're all out celebrating like mad at 10am on days like President's Day and Memorial Day. Celebrating HOW, I shudder to think!
They also seem to assume that the technological revolution hasn't reached us yet and we don't have VCRs to handle the problem.
Whatever the "official" babble is, the REAL reason has to do with money. I'm not sure HOW it has to do with money, but everything about television has to do with money in the end!