The leading hypothesis currently for this is technological development is increasing the value of some skills to a much greater extent then others, this is called Skill Biased Technological Change.
Irrelevant, really. At the end of the day workers in the United States rank 3rd in the entire world in production per hour, and yet, we rank near the bottom of all developed nations in income disparity. No matter what spin you try to put on it, the capitalists are keeping a larger share of the pie today than they were in decades past.
This is a fallacious argument, is every company in every country which has universal healthcare subsidized by the government because it doesn't pay for its workers healthcare?
It's not a fallacious argument at all. There's simply no way to spin the fact that we have a corporation that profits 14B per year paying their employees peanuts while taxpayers cover the gap via safety nets. Wal-Mart obviously isn't the only example, they're just the biggest example.
Labor/capital (IE business owners, those who receive profits via dividends and buy backs) shares have been remarkably stable since we have been measuring them (variance since 1950 is 0.04), if you control for cyclic effects the only divergence of statistical significance was between 2000 and 2008 but has since returned back to the zero line.
Uh, what?
Since Friedman and his ilk pushed the concept of shareholder value in the 1970's there's been a clear change in the way corporations operate.