Can a shopowner in Thailand ban someone from entering their shop on the grounds of their citizenship?
The first tweet, explains the context properly:
I’m at my local hospital this afternoon to get a medical certificate. My work permit expires soon and so I need to run around getting all the documents in order. At the hospital they wanted to check my passport before letting me in to see if I had been abroad recently.
Thailand has been very popular with visitors for decades and if laws existed that discriminated against foreigners this would be commonly known.
So in this case the 'discrimination' probably not because of citizenship, but more about a faulty assumption that the virus is spread by foreigners. This will have nothing to do with Thai law.
The (tweet) OP quotes only another persons claim. The OP states in the first tweet that they checked his passport only to see if he was abroad recently. So he didn't share the same experience of the American. Based on that, this second hand source (that can't be verified) is probably unreliable.
Due to the present (global) uncertainties, caused by the Coronavirus, one should look at the whole picture.
2020-02-04: Coronavirus: Chinese targeted as Italians panic - BBC News
In Italy and elsewhere, panic is spreading much faster than the coronavirus itself. Chinese businesses are empty, shopkeepers are shutting down and Chinese nationals are being targeted.
- At a bar beside the Trevi fountain, a notice was put up banning customers from China.
So the the situation described by the original (tweet) OP is understandable, but the quoted (but not varified) second hand source as well as the events in Italy are not.
The incidents have prompted condemnation from the Italian authorities.
Prime Minister Giuseppe Conte reprimanded the regional governors, telling them that they were not competent to make such a call and that nothing justified such fear.
Discrimination, solely due to citizenship, would be against Human Rights prevention of discrimination.
Special cases may exist for prices that are subsidized and thus only for residents.
Dual pricing was common in the Czech Republic until 1999, when it was ruled illegal (but still persisted). Then a foreign resident had to supply proof of residency to avoid paying the higher price. 2007: Illegal practice of dual pricing persists in Czech Republic
At the time we assumed that this was legal (it was certainly understandable), but it seems that was not the case.
Are the "Human Rights prevention of discrimination" written down somewhere? Also, who enforce them?
How these international laws/conventions are implemented into national laws will differ from country to country.
For Germany they are anchored into the constitution:
Basic Law for the Federal Republic of Germany
Primacy of international law
The general rules of international law shall be an integral part of federal law. They shall take precedence over the laws and directly create rights and duties for the inhabitants of the federal territory.
and are enforced by the Federal Constitutional Court (Bundesverfassungsgericht) and can be passed on to the European Court of Justice (or European Court of Human Rights) should the need arise.
Thailand: Part of Section 30 of the Constitution of the Kingdom of Thailand 2007:
Unjust discrimination against a person on the grounds of the difference in origin, race, language, sex, age, disability, physical or health condition, personal status, economic or social standing, religious belief, education or constitutionally political view, shall not be permitted.