The cost to turn unemployed clerks into construction workers is not close to zero. They would need training, equipment, supervision, health benefits etc. For this reason, when here in Germany someone suggested to get all those unemployed to shovel snow during the winter, the local politicians did not want any such workers - while the unemployed would receive next to nothing, the cost to the community would be higher than having professionals do the job. Forcing people down on their luck to work for free is going to put pressure on the wages of those who do the same work for a living.