The US tax system is difficult for anyone to understand. It is especially difficult when there are US tax "experts" providing advice that is simply wrong.
For example, one expert states: "Disregarded entities exist legally, but they don't have to pay any income taxes unless the foreign-owned SMLLCs are generating income that is FDAP or effectively connected to a US trade or business." The message, then, is that any US income that falls through the cracks between being FDAP (fixed, determinable, annual or periodic) and effectively connected to a US business is not subject to US income tax for nonresidents.
This would create what would seem to be a "loophole" - an oversight by tax writers that really smart people have found to take advantage of. However, Congressional tax writers figured out long ago how to avoid creating loopholes.
Definition of Gross Income for Nonresidents
The definition is in Internal Revenue Code (IRC) section 872(a). Please read this yourself at
26 U.S. Code § 872 - Gross income. To paraphrase, it says gross income of a nonresident alien includes all income (with exceptions detailed later in the Code section) 1) from sources in the United States that is not effectively connected with a US trade or business, and 2) gross income that is effectively connected with a US trade or business (even if not from sources in the United States).
Regulation section 1.872-1(a) puts this rule into user friendly language: "The gross income of a nonresident alien individual for any taxable year includes only (i) the gross income which is derived from sources within the United States and which is not effectively connected for the taxable year with the conduct of a trade or business in the United States by that individual and (ii) the gross income, irrespective of whether such income is derived from sources within or without the United States, which is effectively connected for the taxable year with the conduct of a trade or business in the United States by that individual."
Income from Sources Within the United States
It's clear from section 872(a) that income from sources within the United States is subject to taxation, even if not connected with a US trade or business. The question remains, what is income from sources within the United States?
IRC section 861 addresses that question. Please see
26 U.S. Code § 861 - Income from sources within the United States. Notably, read section 861(a)(6), which says the sale of inventory (e.g., drop shipping) within the United States is US source income. However, if the sale of inventory in the US is US source income, but it is neither FDAP nor effectively connected with a US business, how is it treated? Please read section 864(c)(3) here:
26 U.S. Code § 864 - Definitions and special rules. The paragraph explains that all US source income that is not classified as FDAP income is "treated" as effectively connected with a US trade or business. In other words, it's taxable, but it's taxed at graduated rates like effectively connected income rather than at the flat rate of 30% like FDAP that is not effectively connected.
There are nuances regarding the sale of inventory. The source of purchased inventory depends on where "title passes" and manufactured inventory is sourced where made. But generally, purchased inventory sold to US customers is taxed as effectively connected income, unless a treaty exemption applies.