The fact that the richest Americans are grabbing such a big slice of the pie isn’t exactly breaking news. What is news is that Wall Street is getting nervous about it. Moody’s Investors Service, a private watchdog for the federal government’s fiscal soundness, has concluded that inequality has reached the point that it threatens a system already being strained by increases in the federal debt. But Moody’s also noted that inequality is contributing to slower economic growth, which further aggravates inequality. The high level of U.S. inequality today “sets us apart” from Canada, Australia, and several European countries, Moody’s said in an October report, “Widening Income Inequality Will Weigh on U.S. Credit Profile.” Moody’s central concern is how inequality w…