It's not about the moral gravity but about an illustrative example of a particular category of people who have access to devices where buying stuff is possible - children. Our marketplace has at least some customers that clearly are unable to protect themselves and need protection, e.g. these children. This places some contraints on the properties that this marketplace must have - e.g. reversals must be possible without undue burden. And this places some limitations on what technologies can/can't provide these properties and thus would/wouldn't be suitable.