Currently, foreigners are subjected to 30% withholding tax on dividends when they buy US stocks through a US broker. Will they also be subjected to the same tax if they buy international stocks (say, Hong Kong stocks) through a US broker?
It depends on whether the dividends are distributed by a US entity or not. While companies may be head-quartered in one location, the jurisdiction under which they're organized may be elsewhere entirely. What's important is the jurisdiction under which the corporation is organized (US or not), not where the corporation physically does business.
Dividends : Whether a U.S. or foreign corporation*