I’ve already posted about the general negative attitude/bias users have towards advertising on search engine results pages (SERPs) and their tendency to see, but to broadly ignore AdWords due to multiple possible reasons. As users are more likely to click onto ads that appear useful and not at all distracting, search engines are trying to make Ads on their SERPs to stand out as little as possible to “ensure the best possible user experience“. 
Displaying ads for search engines thus appears to be a delicate trade off between motivating users to click onto the ads and thus creating advertising revenue while at the same time not giving users a (subconscious) negative overall experience (“too many, annoying, misleading or irritating ads”). A negative overall experience will not only cause the user to pay less attention to ads on the respective site, but, in the worst case it might lead the user even to use another search engine for his future searches.
To be more precise:
“users continue to click on advertisements only if the value that a user derives from clicking on an ad exceeds the cost of time required to evaluate the contents of the offer.”
If the value is a large negative number due to e.g. misleading advertising the user will pay less attention to the ads in the future and thus decrease the advertising revenue of the search engine.
As search engines are aiming to provide users with an optimal user experience, it is e.g. forbidden under the editorial guidelines of AdWords to use “call-to-action“-phrases whose only purpose it is to motivate users to click on the ad. Browsing through the net I found a blog today which has found its own way of circumventing AdWord’s editoral guidelines as it has placed an (truly annoying) .GIF graphic file above the ad block that should direct users to the ads, which will create revenues for the owner of that blog.
: Hotchkiss/Garrison/Jensen, Search Engine Use in North America (2005); Greenspan, Searching for Balance (2004); Jansen/Resnick, An Examination of Searcher’s Perceptions of Nonsponsored and Sponsored Links during Ecommerce Web Searching (2006) pp. 1958.
: Guo, User Experience Research an Management of Online Advertising and Merchandising (2009) p. 464.
: Abrahams/Schwarz, Ad Auction Design and User Experience (2008) p. 98.