However, under certain circumstances, payment for personal services performed in the United States is not considered income from sources within the United States. For example, personal services performed by an independent nonresident alien contractor specifically exempted by a tax treaty. For more examples, see the Pay for Personal Service section in Publication 515, Withholding of Tax on Nonresident Aliens and Foreign Entities.
Portfolio income can come from multiple sources – interest/bond coupons, stock dividends, financial strategies including derivatives and capital growth.  Each offers some cash flow and some also offer potential capital gain with some risk from liquidity and volatile prices.  In this low-yield environment, many investors rely too heavily on cash flows and to pursue this are venturing further into riskier areas than they’d normally consider.