There is a constant battle going on between parking providers and advertisers on the one hand and search engines (not necessarily public search engines) on the other.
The search engines are trying to gather literally as much information as they can either about certain types of domain names or even general domain names. To the owners of these search engines information is money and power. With the information they gather with click bots they could predict spending patterns by advertisers (as individual advertisers, as a genre of advertisers, and even whole industries). It is this kind of information as well as seeing what domains may be for sale, poorly developed, under monetized with regard to advertising, and even who is likely to own portfolios (similar parking/layout/advertising/etc.) that is such valuable information to these search engine companies.
Parking providers such as Yahoo and Google are constantly in battle with such search engine companies, for a start they can crush an advertisers advertising campaign by producing false click throughs without of course any likelyhood of a benefit to the advertiser. Also it allows for the 'parking providers' confidential commercial information to be accessed by external agencies. This is obviously a situation that parking providers fight back against. As soon as a click bot is identified the parking providers aim to block its access to their information, however it is so relatively easy for the search engine company to alter the computers address or tweak the search in such a way that again the same search engines again gain access to the network.
This is what is happening 24 hours a day every day of the year. Indeed it is happening whilst I write this post. Many times in the past low penetration by the click bots went undetected, however, now with the realization of the problem by the parking providers, especially with companies like Parked whereby multiple clicks by the same user are paid out the eratic nature of the click throughs does seem bizaar. Unfortunately it is something we as domainers have to live with.
Of course there is the option of traansferring your domains to a parking company that only pays out for the first click through, but then again we all know that such companies are generally robbing us blind. So to sum up, it is an inconvenience that we have to live with, it is not Yahoo's fault, it is not Parked.com's fault, it is those damn Search Engine companies who no matter what will continue to search for such information and utilize such information commercially.