"Labor" is not a lumpen, indistinguishable mass negotiating with an equally indivisible "capital." Individuals workers bargain with individual employers, unless aided by collective-bargaining laws, which artificially forces together their otherwise dissimilar interests. The variation within "labor" is much greater than the differences between Mr. Stiglitz's imaginary categories of "labor" and "capital," which is Marx's language.
Joseph Stiglitz, one of whose most popular books is called Whither Socialism?, is a very smart man in the ways of academic economists, but a socialist through and through.
The left is really writing a lot of columns about leftist class warfare ("the rich are getting richer", "there is a widening gap between the rich and the poor", "the economy is not fair", etc.). It seems to be gaining some traction with the ignorant.