Can US nationals use citizen line?
 

I'm intensely familiar with the status of non-citizen US nationals. They are mostly American Samoans, maybe a few associated with the Northern Marianas. They have the same US passports, except in the observation page there is a note that they are not US citizens. Foreign governments only care about nationality, not citizenship. US citizenship seems to be a purely domestic idea. It's in the consitution, and although not prevented to do so, no US jurisdictions outside of AS allow non-citizen nationals to vote.

I've just been asked by someone something I never thought of: At US airport POEs, the line says "citizens". Can US non-citizen nationals use that line? What about "US Citizen Services" at embassies and consulates?

My immediate "guess" would be "yes" for both. At airports and embassies/consulates, there's nothing preventing them from servicing all nationals. However, "citizen" is a much clearer term (national can be an adjective). Few citizens know they are also nationals. So at both places they practically have to use signage with the word "citizen".

However, I can't seem to find anything on the internet to confirm my "guess". Not even US Embassy Apia. Does anyone have real-life accounts of these situations?

He/she can use the citizen line, since green card holders also use those lines. Being a national-only is in the legal area between a citizen and a green card holder.

Given that Canadians can use the US citizen line, I'm sure us nationals can.

(There is a chance that I am wrong and that there is indeed a regulation or rule that specifies what lines will exist and who may use them at ports of entry, but I sincerely doubt one exists); the line for "Citizens" is not something that is prescribed anywhere - the separation of types of persons seeking entry into the US into various lines is simply something that is done for administrative and practical convenience as it engenders efficiency in processing requests to enter. Therefore, since US nationals travel on US passports, and their processing for entry into the US is no different than for US citizens, US nationals may use the US Citizens line. Or, just as US citizens may, US nationals may use the non-citizens line.

US National, US Citizen and a legal permanent resident are three distinctly different categories and one should not infer that "national" is in some "grey area" (my paraphrasing of the above comment) between citizen and LPR. Chapter 7 of the Foreign Affairs Manual is quite an interesting read for on the meanings and implications, at least regarding being abroad. https://fam.state.gov/FAM/07FAM/07FAM0010.html

As Section 107 says, it is probably just a matter of nomenclature.

They could easily relabel the lines to read "US citizens and nationals" or even "US passport holders".

The UK has a similar problem. The immigration desks are divided into "UK/EU passports" and "non-EU passports" but this is a horribly inaccurate description of who should be queuing up where.

For example, Norway is not in the EU, but Norwegians can use the UK/EU passport queue. British National (Overseas) passports are UK passports but they must use the non-EU passport queue.

But there is no actual regulation on this separation, so immigration officers must process everyone who steps up to their desk even if you were in the "wrong" queue.

More accurate labels are found in the Schengen area, which labels the booths as "EU/EEA/CH passports" or "All passports". This correctly excludes certain British passports such as Channel Islands ones where holders do not have EEA treaty rights until they have lived in the UK for 5 years (but this may become moot in 2019) - so they are not "EU passports" until that is the case.

For the UK, better labels would be "British, EU, EEA and Swiss citizens and their family members" and "Everyone else" although this still excludes people such as Australians who have right of abode in the UK (with a label certifying such in the passport), who may also use the UK queue.

Aren't all US Nationals for all intents and purposes considered to be US citizens without the right to vote?

Interesting question would be if someone was a US national and also a citizen of the recently-banned countries. Would they be equated to a green card holder and not be allowed back into the US?

Green cards can only be issued to aliens and US nationals are not aliens.

Is that really the case? In Poland there are always "EU/EEA/CH" line and then there is the "All passports" line as you mentioned, and if you're Ukrainian on the land border with Ukraine and step to the EU line, you're not going through. Is that just country by country regulation?