This is how the US retains Wall Street as the prominent financial center of world commerce. It should be noted that the UK has a similar policy of not taxing non-residents who make gains on stocks held in UK broker accounts. The instant any country even hinted it would levy capital gains taxes on non-taxpayers, their capital would flee to some other tax-exempt jurisdiction.
The issue is tracked here in the US by the Center for Freedom and Prosperity Foundation, at www.freedomandprosperity.org