It's just one of a number of possible business models. Photographers who charge a sitting fee are establishing the minimum cost of a sitting as Mike says, apart from prints, and then they make most of the profit from prints. You might offer a package that itemizes the sitting fee, proofs, and a certain number of prints, and then offer additional prints at additional cost, or offer different packages, say with a discount for more pre-ordered prints, as opposed to prints ordered later.