One answer, hinted at in the film version, is that the clones are under constant surveillance. This is shown several times when they enter or leave their dwelling and run their wrist past an electronic scanner which tracks their whereabouts. It is suggested that the thing being scanned is encoded into their bodies perhaps a tattoo or implanted chip. This surveillance implies that they'd never be able to escape their fate. The surveillance also mirrors the current situation in Great Britain, whereby closed-circuit cameras have been installed in many public places and a number of activists have sought to protest the invasion of privacy by blocking or destroying them. This reference to the reality of modern day Britain carries the implication that the creation of clones to provide organ donations is also a plausible possibility.