This article is ancient (2008), which puts it right at the start of the reform that became Obamacare. It's a pretty good explanation, or starting point.
Quote:
In 1942, the US set up a National War Labor Board. It had the power to set a cap on all wage increases. But it let employers circumvent the cap by offering "fringe benefits" - notably, health insurance. The fringe benefits created a huge tax subsidy; they were treated as tax-deductible expenses for corporations, but not as taxable income for workers.