If you're a site owner and you're using bots to screw over your advertisers, that's one thing.
But otherwise, what's their claim here? I'm sure web crawlers interact with ads all the time in the course of their crawling, and no one's complaining about that.
That's not a real, legal, thing. "Click fraud" is just what some businesses term interactions that they might not pay for.
There is no legal, or moral, requirement to only click links if there is some chance that you will increase the income of some "advertiser".
Though the harms it causes are similar:
* advertiser is paying for clicks/impressions from a bot rather than a user
* publisher/network sending lower quality traffic to advertiser faces reputation and financial risk
Likewise, depending on the situation fake social media profiles also can harm the quality of said network.
In terms of damages caused by the researchers' action... clicks would be the most harmful. Especially against insurance, loans, and mortgages which they seem to be interested in. Those have some of the highest CPCs around. (e.g. http://www.wordstream.com/articles/most-expensive-keywords)
Personally I think it's a little unlikely, but criminal prosecution can all be very capricious and dependent on local and regional priorities. And so much the better that their suit against the US might bring about limits of CFAA via precedent.
If I properly quote Wm Shakespeare “You speak an infinite deal of nothing.” (Merchant of Venice) then within various limitations designed for academic quotation use, I don't owe ole bill a penny nor do I need a copyright license for that academic purpose quotation.
On the other hand if I were a musician and sampled 2 seconds of a song, supposedly I need a legal release or the owner of the sampled song will have our government give me the economic death penalty in the courts.
The CFAA is kinda weird legally having the infinitely powerful government team up with the infinitely powerful megacorporations just to make sure individuals follow arbitrary corporate defined local rules of behavior. I'm having trouble finding a non-CFAA example in the real world. I guess it would be like if a shopping mall privately published a rule about not saying "merry christmas" for inclusiveness reasons, then the local police shoot in the head any private citizen at the mall caught saying "merry christmas".
Next thing you know they'll be running extension cords over your fence and into your windows in order to power their experiments for academic purposes.