If Forbes set a minimum price for their ad backfill inventory (the shitty ad inventory they fill with low quality ads), could it economically not make sense to the networks to fill that space with malware ads?
Malware ads are being displayed because they are winning the auctions. That's because - on a CPM basis - malware authors are probably the ones who will always be able to pay the most. A successfully infected machine is worth a good chunk of change.
Ad networks are uniquely suited to malware delivery. Have an exploit which only works for OS X users? Don't bid on the windows users.