I think that's one of the points though. Big sites can back up their digital reach claims such as by using third parties. But if vetting yourself for advertisers was to become a requirement, advertisers just might not bother with small sites. Though admittedly Google makes money off small sites--but I bet most of those small sites don't make enough to make note of.
Right. Which is the point both of us are making I think. If you can't do some minimum floor of fraud detection more or less transparently and cheaply, you fall back on more heavyweight mechanisms and probably screen out any site that isn't "big."