That concern exists with 3rd-party ad networks as well: a publisher site could certainly cause ad impressions to happen without an eyeball actually seeing those ads. (e.g. text-indent: -5000px)

The answer is just that they don't, but they do know if they are getting conversions. So the market corrects itself, and after some amount of time advertisers will (probably) stop bidding for impressions or clicks that aren't doing them any good.

Also, falsely indicating to advertisers that you are showing their ads is fraud.