Socrates' argument here is essentially that, since children in their innocence may be unable to discriminate between the good and the bad in artistic portrayals of these qualities, there is no good reason to permit children a choice in their formative years insofar as their training in the beauties of "music" is concerned (Plato and the Greeks generally classified literature as a form of music). Permitting the children and maturing young adults a choice in the matter of good and bad in their taste for the arts simply introduces an exercise in liberty that does nothing to advance the cause of the state.
We have in our own time witnessed a continuum of this debate of morality vis-à-vis the arts and whether the state is obliged to support artistic enterprises of questionable moral worth.
