Most people are familiar with Google AdSense, which produces the ubiquitous column of text ads that appear on just about every web site that's trying to scrape up some advertising dollars. The flip side of this service is called AdWords, where Google takes money from advertisers so that those advertisers appear in Google search results and on AdSense pages. What most people don't know is that Google is perfectly willing to take money from AdWords advertisers selling things that they prohibit their AdSense publishers from publishing.
Become an AdSense publisher and Google will put ads on your site and you might make some money -- just as long as you're not writing about things disapproved of by Google staffers. (Check out Google's rules regarding Site Content.)
Despite the fact that viewing porn, discussing drugs, cussing, selling guns and knives, drinking beer, consuming liquor and taking prescription drugs are all 100% legal in the United States, Google won't let your site participate in their AdSense program if you discuss or publish information about any of these things on your web site. For now they'll still index your site so that people can find it with the Google search engine, but they make it very clear that do not like you and don't want to do business with you.
However, if you search for words relating to these subjects you'll find that Google has lots of advertisers (AdWords purchasers) who sell these very things. (Check the links above and see.)
You'd think that since Google has advertisers that are selling porn, that those advertisers might want their ads to appear on sites that cater to people interested in porn. You'd think that since Google has advertisers that sell hunting rifles that those advertisers might want to appear on sites catering to gun enthusiasts. You'd think that people advertising any of the above items would find a larger audience on sites discussing their the very things that they're selling, but since Google disapproves, the advertisers can't reach their target audience and their target audience can't monetize their web sites.
Google also prohibits publishers who sell or promote beer or hard alcohol, but apparently wine is OK. Google's prohibition against tobacco-related products apparently extends to both advertisers and publishers -- I couldn't find any ads for pipes, cigars, tobacco, or cigarettes.