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Most of them turn out to be e-gold/ecurrency, the latest $1000 pyramid scheme or data processing. In my case it seems to be coming from scam-alert and gurubusters justicereviews and a few others. So far google hasn't banned them even though data processing and e-gold offers are in violation of their own policies. Maybe google hasn't seen the child porn charge against e-gold? Seems really odd that they would allow this at all.
That being said, I've seen plenty of these ads too. None in any of my clients' industries, but I know the ones you speak of. I don't like them any more than you do in most cases, but I don't think the problem is widespread or sinister enough to generate the volume of complaints from advertisers that would warrant a policy amendment.
The only relevancy is that they scream scam and place a some type of opinion on their site, beyond that there is nothing except "buy my scam that has nothing to do with what you were searching for". Notwithstanding the fact that these "opportunities" are prohibited in Google's guidelines...