Google has been getting tighter with their control of their ads, fighting arbitrage with the following changes

• manual reviews, smart pricing discounting (of publisher clicks), and quality scores (increasing the cost of advertising junk)
• improved duplicate content detection
• decreasing the clickable area in many AdSense ad units
• killing sub-syndication of their feed with companies like Ask
• keeping a greater percentage of ad revenue from each click
• requiring advertiser display URLs to match ad destination (starting April 1st)

From this graph you can see the sharp rise of click arbitrage quickly fell off when Google decided to work on fixing the problem.

Yahoo! has turned a blind eye to arbitrage for years. Some people who had strong feeds only saw their arbitrage profit margins increase as the weaker players could no longer compete arbitraging Google traffic. And then Yahoo! dropped a bomb, announcing that arbitrage to their ad feeds is now against their TOS and Parked.com confirmed it.

The biggest distributed ad networks do not want to buy any traffic that is bought directly via similar networks. These companies are cutting their short term revenues in an attempt to make their ad ecosystem healthier. On multiple occasions I have logged into Yahoo! Search Marketing to see a random multi-hundred dollar spend on a keyword that typically costs under $1 a day. I, for one, am glad to see this crap die. But it died a few years too late.

Google and Yahoo want a direct relationship with publishers and merchants. They hate virtually any type of affiliate that is not a highbrow relationship. More power to the organic players, and please static text links instead!