We have reached the point where any type of link building that Google encouraged over the past 10 years is now penalized. In the case of natural links, I don't believe that Google's algorithms can determine what is a natural link.
Google is now believed to be actively penalizing sites that pick up too many links from a small group of forums, which includes signature links. They have reached the point of interference with the way the Internet is intended to operate.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin