Wow, so they didn't even accuse you of fraud... just "pre-fraud". It's especially funny since every active AdSense account is one malicious visitor away from "fraudulent activity". I'm not sure how you can be more "at risk" than that.
I worry sometimes about the potential for malicious drive-bys. I've thought about trying to set up some code that registers when someone clicks on an ad and and then shows a non-ad in its place for the next X page loads to try to prevent that sort of thing. Not sure if it would work, though.
AFAIK you're not under any obligations to show Adsense ads to all users, or all of the time. I know quite a few people who only show Adsense ads to search engine visitors, for example, or block ads for people arriving with certain referrers.
This seems like a similar move.