It's not the same as ads - the guidelines only refers to paid links which pass PageRank (ie. Without nofollow on them).
Doing something to achieve an SEO benefit that is in contravention of the guidelines of the search engine you are optimising for, because you assume you can't or won't be caught (for example because they can't look at your books and weren't present during the meeting where you sold the link) is the definition of black hat SEO.
It may well be the case that Google lacks the ability to enforce certain aspects of their guidelines at some points in time, but as we have seen in the past couple of years when they do figure it out it can be a business closing event for some.
Much better to just follow the guidelines don't you think?
Doing something to achieve an SEO benefit that is in contravention of the guidelines of the search engine you are optimising for, because you assume you can't or won't be caught (for example because they can't look at your books and weren't present during the meeting where you sold the link) is the definition of black hat SEO.
It may well be the case that Google lacks the ability to enforce certain aspects of their guidelines at some points in time, but as we have seen in the past couple of years when they do figure it out it can be a business closing event for some.
Much better to just follow the guidelines don't you think?