If, if someone sold you a single link on a page of editorial content that was somewhat relevant, you'd have a "natural" looking link that was paid. Most are too lazy to do that though, because that would also mean really thinking about a way to make the link useful!
See a difference?
However 1) it's too much work and 2) sites would have to actually screen their advertisers and only accept those that they could make work with the site in a way that didn't make them look too stupid.
You are right, I suppose that people that sell and purchase links on sites don't give it that much thought. Therefore, a true natural looking content link as you describe above would probably be very hard to find in a purchase type of situation.
Since you brought it up, what is with all these site footers I am seeing lately? Do sites pay to be in a sitewide footer? Drives me nuts. I find a potential link exchange partner and they have a nice directory with all relevant sites. Then, I take a look at their footer and they have gambling sites, medication sites, etc. Why oh why?