Presumably much of the detection is probabilistic. Some algorithm notices a bunch odd traffic from Chile hitting a French website, clicking ads at a high rate,

Presumably much of the detection is probabilistic. Some algorithm notices a bunch odd traffic from Chile hitting a French website, clicking ads at a high rate, so it all gets tagged as fraudulent. But some of that traffic was probably real. If the fraud rate is high enough, they likely can't pick out any real users anymore, as they get lost in the noise.