Traditionally, link building has been a major part of any SEO campaign. The more links you have, the better you'll rank in the search results. And sadly, that was the case for many years. Many SEO companies offered link building packages, and they no doubt still do today, but in order to keep clients happy the emphasis had been more on quantity than quality [I'm speaking from experience here, but I know there are some great SEO companies out there too]. As an SEO company it was a difficult position to be in. Building links in a natural, organic way is hard work and time consuming, something that some clients may not appreciate. Although I see clients becoming more aware of SEO techniques, I think it's really up to the SEO company to educate clients as much as possible and just be open and upfront about what's involved.