Notice that most of the affected advertisers are the ones running the CPM campaigns, not the CPC campaigns.

The defrauded CPC campaigns were directing people to sites with video content that were running mainly display advertising campaigns. These display ads were mainly from brand advertisers that do not care too much about clickthroughs or conversions (e.g., Coca Cola, or Continental, or Verizon, or ...).

So, even though the guys running the CPC campaigns (e.g. Mevio) were victims of fraud, they were getting the (invisible) traffic and they were selling the display ads to this traffic. Not that Mevio et al. do not care. Quite the oppositve. However they were just getting more traffic. Why would you research your own CPC ad campaign that is effective in bringing you traffic?

I tell you, the scammer has executed this beautifully.