Each use of a pesticide on each crop requires a unique registration. In other words, the pesticide named Captan must be registered for use on onions, and it must be registered a second time if it is to be used on peaches. Under common law, putting poisons into your neighbor's well, or onto your neighbor's food, is considered very anti-social and is strictly illegal. Their primary purpose is to ensure compliance with good agricultural practice. In other words, the first concern in setting a tolerance is to allow enough of the pesticide to be used to kill the target pests.
Health is secondary.
EPA's tolerance for methyl parathion is 41 times as high as the RfD for methyl parathion. Furthermore, RfDs are set to protect adults, not children. The EPA has never set an RfD or a tolerance based on children's health. Thus if the National Academy's recommendations were to be carried out, all pesticide tolerances would have to be reduced by a factor of 10 in an effort to protect children. It seems safe to say, therefore, that no legal levels of pesticides can be considered safe for children, and many legal levels of pesticides are clearly not safe even for adults.