We do know that within the link building industry there are many who follow these rules but that there are probably just as many who bend these rules.
If Google catch you then you will be penalized, the problem as we see it is that Google aren't that good at catching people and even if they do then it is very hard to prove beyond doubt if a company has acted intentionally to deceive. With these grey areas expanding all the time many companies are prepared to take the risks asa they are actually very small in comparison to the potential gains.
Google's RulesGoogle and most other search engines use links to determine reputation. A site's ranking in Google search results is partly based on analysis of those sites that link to it. Link-based analysis is an extremely useful way of measuring a site's value, and has greatly improved the quality of web search. Both the quantity and, more importantly, the quality of links count towards this rating. However, some SEOs and webmasters engage in the practice of buying and selling links that pass PageRank, disregarding the quality of the links, the sources, and the long-term impact it will have on their sites. Buying or selling links that pass PageRank is in violation of Google's Webmaster Guidelines and can negatively impact a site's ranking in search results. Not all paid links violate our guidelines. Buying and selling links is a normal part of the economy of the web when done for advertising purposes, and not for manipulation of search results. Links purchased for advertising should be designated as such. This can be done in several ways, such as: