It's been years since I found that info, so I cannot quote you chapter and verse. It may, in fact, be part of the FCC Rules & Regs (as is the requirement that domestic SW stations run a minimum of 50kW), or it may be part of the ITU Conventions to which the USA is a signatory.
I'd be willing to bet that if you tried a Google search you could find the source of that info. If you can't, I'd like to know that I'm wrong (it does happen, you know). [Or maybe not; I just tried my own suggestion and found nothing.]
Yes, the Canadian domestics do run ads, but they are "domestic" stations relaying MW xmtrs and are not truly intended for international listeners. Radio Thailand may consider their transmissions to be domestic, or they may not be a signatory to the Conventions.
America needs a sense of humor...
Edited 1 time(s). Last edit at 10/28/2014 12:11PM by jtart.