Capital gains and dividends are usually taxed at lower rates than regular income. This can be easily changed with legislation.

However, the billionaires work their offshore accounts to much more "legal" tax minimization advantage. For example, if a billionaires owns a holding company in Bermuda and that company buys stocks on the New York Stock Exchange, the billionaire will pay the Bermuda rate for whatever profits are gained. And the Bermuda tax rate is very low.