They create the wrong incentives. In a niche I operate in, there are a ton of otherwise reputable websites promoting inferior products merely because they make

They create the wrong incentives. In a niche I operate in, there are a ton of otherwise reputable websites promoting inferior products merely because they make more money promoting those products rather than products whose owners don't have an affiliate system. A widget hand-made by an owner-operator in small-town USA who sells from his own website gets ignored while the Amazon-listed widget made in a Chinese sweatshop gets shilled because Amazon pays affiliates.