I've actually bought luggage at those airside shops, not once but twice. Once to replace a bag that broke on the way to the airport, and the other time to consolidate 3-4 shopping bags (which went fine through security but would have been a problem boarding the flight). Prices were surprisingly reasonable (maybe $20 premium over what the same bag would have cost outside the airport). So if you travel enough it will eventual become necessary, and it isn't as absurd as it sounds at first blush.