I thought I explained that in my posting: because middle class parents have other means of correcting their children's behavior at their disposal (namely withdrawing the privilege of conspicuous consumption), which the working class parents often lack. This reasoning rests on the assumption that individualistic exemptionism from social rules sucks, children must be taught responsibility and respect for others, and if they refuse to learn, they deserve a corrective action. Hence it is OK to use physical punishment if other means are unavailable or do not work.
wojtek