Atlanta's a major air travel hub, so extra-cheap flights are partially a result of that. If the poster above you has parents that are off the beaten path, it's very likely those $49 deals don't exist for the flight she or he needs.
It's simply demand in the holiday case, but it's still ridiculous. At other times, high demand does not necessary apply and they do all kinds of things to generate demand and compete for butts in seats, especially when fuel prices are low.
But with a high-demand holiday, at a hub almost entirely monopolized by one airline, there is no ceiling to how high they can raise price in response to that demand.