There aren’t any strictly defined policies on SEO, but all search providers to supply webmasters with resources to help them build and maintain the best website possible.
The policy of Google, Yahoo, and Bing is that they are businesses who want to grow their market share by establishing a reputation for delivering the best and most relevant search results to their customers (internet users) in the form of high quality content. As a result, the resources they provide all point you towards creating as user-friendly a website as possible.
Search providers aren’t stupid, and they know that SEO happens. However, they’re tolerant of a variety of SEO practices, including link building, and will usually only take action against sites when they fall below certain quality standards or they are making blatant efforts to manipulate the search algorithms.