There must be ways for Google, Yahoo etc to tackle this but whenever we've approached them its been a long drawn out process with us having to provide significant proof. We stopped advertising on content networks in the end as it made virtually no difference to our sales.
I contacted G once when I noticed that someone had clicked on about 8 pages in a couple of seconds. I had a simple question: "did I pay for this click?" They were unable or unwilling to answer. I've been suspicious since.
And I second your observation on content networks. It seems to be largely without useful controls.
google / yahoo probably make a good bit of their turnover from fraudulent clicks. I would suspect it to be a bit more than the percentage quoted as well...
If the actual number would be measurable somehow I would expect the bottom to drop out of keyword advertising overnight. Those advertisers savvy enough to do a/b testing on google adwords on google, the channel, both and none have already found out that just advertising on adwords on google is a very large portion of the conversion, it depends on the product whether or not it is worth to go after the little bit of extra sales that can be had in 'the channel'.
The lack of transparency at google is another very large part of the problem here, if they would make public their 'take' and the percentage of fraudulent clicks removed then that would do a lot of good for the market. Even if it would probably hurt Google's bottom line.
Advertisers can also track click sources, and eliminate those with lower-than-average conversion rates. There are plenty of reasons they would want to do this outside of click fraud.
