Reading an article about one of the click fraud lawsuits (http://news.yahoo.com/s/nm/20051208/wr_nm/media_google_lawsuit_dc).
This was mentioned:
Click fraud is not "fraud" as defined under the law. Rather, it is an industry term used to describe the deliberate clicking on Web search ads by users with no plans to do business with the advertiser.
Rival companies might employ people or machines to do this because the advertiser has to pay the Web search provider for each click.
I believe that this is fraud regardless of anything. If a competitor is depleting or running down my spend on purpose, it is fraud! But I also agree with rfontaine, that plenty of people do click on ads just "to find out more information on a topic of interest".
This has been my non-lawyer take on the legality of click fraud.
Fraud is when you profit or stop someone else from profiting. Hence, clicking on your AdSense ads over and over is click fraud; however, it's questionable if it's legally defined fraud until you cash the check (i.e. have profited). It could be that it's just easier to prosecute with the proof that you made money from the deception, and thus all the lawsuits have waited until that point in time.
You are suppose to click on a search result on Google.com - so if you choose to click on the same listing over and over, it's not really fraud - you're just clicking on an option and NOT directly making money from it. Of course, one could argue intent and ruining someone else's business - and that gets into lawyer speak (which is over my head).