Treating a few foreigners for free might actually be cheaper than setting up a billing system.
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.
If a country has cheap treatment though, they could earn a lot through medical tourism.