We're adding the option soon. For now we just wanted to get the site out there so people could see the savings they can receive if they use Bitcoin. Having the prices set in USD helps consumers compare across sites much easier.
I agree, the price should not be shown in dollars, otherwise I feel it kind of defeats the purpose. But this could be the beginning of something great. Bitcoin needs a real economy behind it to sustain itself and to become a lot less susceptible to speculators.