Informal Click Fraud (aka Click Fraud as tip jar)

The blog writer at FrozenNorth.org has posed an interesting variant of click fraud, which he describes as an electronic tip jar. Before AdSense, some web sites used to encourage visitors to click the ads and visit the advertisers but as this contravenes the AdSense ToS, a publisher cannot openly encourage clicks. I don't see such use of AdSense as being any more of a tip jar than, say, its use on any other publisher page.