The search engines will let you have and keep any backlink you want and that is why people keep doing it. However, the problems come into play when people abuse the methods and services by over posting, pinging as Frankie mentioned and then a couple of other factors coming out of the woodwork.
The penalty comes in the form of devalued rank, authority and even keyword position. These things all come down to simple ' content relevance '. IF the links don't hold true to their common relation to your site's niche market, you are going to be discredited on every unrelated link, taking so many positives to keep you above water.
That's why I suggest that you do your homework and either build your own links from link wheels and proper seo promotion, as well as natural, content relevant link building.
If someone is using a crappy little site that they are continuing to spam, cpa and scam promote through, then it doesn't matter, as they will never want to put in the time and effort of a real, high ranking authority site. It is too easy to throw up another trash site and make a few bucks that they throw away half of to make the next one.
Those sites can use whatever means to build because they are on a short lifespan anyway and need to make whatever they can before getting shut down. By the time google or the other search engines degrade their stats to levels of no use, the owner has already made a couple of hundred dollars and has moved on.
The problem with that is that in doing so, they are forcing google's position to become more and more strict on sites, in an attempt to crack down on it and are furthering the devalue of potential generalized links that may actually end up being of relevance to a site or service that works as a directory or recommendation site of sorts.
Keep it simple and don't fall into that ' get rich quick ' type of thing, as building your links properly will take far less work, effort and will bring you greater rewards to your site's standing over time, building it as an authority.
Google is in the ' information ' business and content is the job user-generated source of that relevant material they use. It is simple to rank in google for the top spots if you simply give them what they want. A bunch of junk, non-relevant backlinks is NOT it.