IANAL but I doubt they’re banned on websites. This is simply because it would be so hard to police that companies wouldn’t bother.
And you’ve got the whole site hosted in Country X but viewed in Country Y problem. Advertising on the web doesn’t seem to stick to the same rules that apply for advertising elsewhere. There may be some sort of internal code of conduct but I doubt all advertisers stick to it.
I’d doubt any reputable company would do it because a) it doesn’t really work (see above) and b) someone would spot it and start an outcry, an e-mail petition, a mass boycot of the company and some pro-active computer dude would probably hack their site in response.
Of course a company might do it for this reason, but here the real advertising is coming from the generated outcry and coverage (which is free) than from the subliminal messages.
We could, of course, design a site with two big red buttons one with a subliminal ‘push me’ in it and measure which one gets pressed more. Hmmm, that’d be quite fun actually I might add that to my list of things to do when bored one day.