This is a concrete example of an intuitive fact. There is a limit to how much a government can tax the "rich." This limit will be the tax differential between locations, investment vehicles, or over time, plus the cost of moving the money.<p>An example of tax rate differences by location is Michael Arrington: California vs. Washington (<a href="http://news.ycombinator.com/item?id=1736265" rel="nofollow">http://news.ycombinator.com/item?id=1736265</a>).<p>Ballmer's sale appears to be the result of a time-based tax rate differential. IRAs are also time-based tax rate differentials: the premise is that the investor defers paying taxes on the IRA investment until he retires and (he hopes) is taxed at a lower rate.<p>Examples of investment vehicle-based tax differentials are tax free (government) bonds, Roth IRA (interest is tax-free), special "economic investment zones", etc.