One of the guidelines listed at Google is avoiding sneaky redirects. I've seen a number of articles smugly stating one only has to remove sneaky redirects to fix the problem, but without ever researching what a redirect is.
What is a sneaky redirect?
Forget what Google says publicly. That's about as accurate as Google's public list of backlinks to your site. Instead, find Google's confidential "Raters Guide" (March 11, 2011, page 100) which was leaked to the web. It discusses what a sneaky redirect is, what it looks like, what it does, and how to find it.
You know what a sneaky redirect is? An affiliate link.
The example in the Raters Guide uses a Commission Junction link. The website has a link to CJ which redirects the user to the merchant (JC Whitney in Google's example).
Google also says these sneaky redirects are common when taking people to Amazon, eBay, Zappos, etc.
It is ok to directly link to a merchant. But it is "sneaky" if you link through CJ which "redirects" you to the merchant. That is spam as the link is intended to make you money, and Google has made clear it considers the vast majority of affiliate sites to be spam because they do not add sufficient extra value. Whatever that is.
Bottom line: It is easy enough to say Google is targeting "web spam." How awful. Who isn't against web spam?
... Until you realize how Google defines web spam.