lrn2CapitalGainsTax. By taking a low salary and earning the majority of their wealth in dividends the ultra-rich can avoid roughly half the %'s that the rest of us are taxed at. Sam Walton didn't take a ridiculously low salary because it was cute or because of some nebulous sense of company pride. He did it for the tax breaks. That's just the beginning too, there are many tricks that the rich can do to pay much lower percentages that would make the rest of us balk. (setting up charities, shell companies, and even flat-out avoiding the aforementioned capital gains tax are a few perfectly legal ways to lower your taxes)
Trump doesn't want people to see what % he gets taxed at because his opposition will accuse him of avoiding taxes. When in reality, we should be much, much more concerned about his mental stability it he were not taking advantage of these legal tax loopholes.