What the author is trying to put across in this passage is that

  Is work for prisoners a privilege to save them from the demoralizing effects and misery of endless unoccupied hour? Is it something added to a prison sentence to make it harder and more unpleasant, or something which should have a positive value as part of a system of rehabilitation?
Those magistrates who clung to a sentence of hard labour doubtless looked upon strenuous work as an additional punishment. This point of view is widely accepted as right and proper, but it ignores the fact that unwillingness to work is often one of the immediate causes of criminality. To send prisoners back to the outside world, more than ever convinced that labour is an evil to be avoided, it to confirm them in their old way of life. It has been said that the purposed of prison work in a programme of rehabilitation is twofold: training for work and training by work. The prisoner, that is to say, needs to be trained in habits of industry; but over and above this, he will gain immeasurably if it is possible to rouse in him the consciousness of self-mastery and of purpose that the completion of any worthwhile piece of work can give to the doer. He may find the pride of achievement in something more satisfying, and more socially desirable, than crime. But these things can only come when the work itself has a purpose and demands an effort.