What if they're not an American citizen? What if they're also in a different country?
Quoting Wikipedia:
The European Health Insurance Card (or EHIC) is issued free of charge and allows anyone who is insured by or covered by a statutory social security scheme of the EEA countries and Switzerland to receive medical treatment in another member state free or at a reduced cost, if that treatment becomes necessary during their visit (for example, due to illness or an accident), or if they have a chronic pre-existing condition which requires care such as kidney dialysis. The term of validity of the card varies according to the issuing country.
The intention of the scheme is to allow people to continue their stay in a country without having to return home for medical care; as such, it does not cover people who have visited a country for the purpose of obtaining medical care, nor does it cover care, such as many types of dental treatment, which can be delayed until the individual returns to his or her home country. The costs not covered by self-liability fees are paid by the issuing country, which is usually the country of residence but may also be the country where one receives the most pension from.[1]
It only covers healthcare which is normally covered by a statutory health care system in the visited country, so it does not render travel insurance obsolete.
https://en.wikipedia.org/wiki/European_Health_Insurance_Card
If it's not clear, my tongue is firmly in my cheek as I say that, but where it isn't in my cheek is when I'm telling you some people don't just buy your simple pro-universal-healthcare argument wholesale, especially when you refuse to defend it against valid questions like this.
Universal healthcare would be great if it was cheap and covered everything.
I'll offer some advice, although I doubt you think you have much more to learn in life. If you're walking around looking for donations or support towards a cause, perhaps mocking those who may not share the identical philosophical beliefs as you isn't the optimum approach.
Treating a few foreigners for free might actually be cheaper than setting up a billing system.
If a country has cheap treatment though, they could earn a lot through medical tourism.
Had a visit with a GP, he sent me next door for Digital X-Rays, and then I went back to to the GP where he looked at them and sent me home with a brace (no break).
GP visits: $25 X-Rays: $25 Brace: $15
I don't think I ever ended up submitting the receipts to SaskHealth... Just wasn't that big of a concern :)
The US has no such deals because it does not have a UHC system to deal with. However, most UHC systems that do not charge aren't going to pay out for setting up a billing infrastructure for the occasional American. The ones with co-pays etc already have a billing structure in place, and may charge you. Or not, it would be pretty mortifying to have to charge someone for health care.