I have read over the years Matt Cutts (Google Guy) say that as long as you don't get paid cash other things that would have the exact same indication whether a link should be trusted are not penalized. So if you bought the entire site and then added links to link to you (as large companies can do) that is fine (so if some large company buys a small company and then links to the corporate parent). But paying a fee and getting links is not.
Similarly if you pay a speaker to speak at your conference and they add a link to your site I think Google says is fine. Or if you win a scholarship and you link to the sponsor or the school from your site that is fine. And in my opinion all 3 of those obviously should be fine. They provide valuable insight I would want to include if I were deciding what are relevant links.
Google is trying to collect useful data from the web to make judgements for search rankings. They decided that to make their jobs easier it would be better to stop some forms of influence in adding links. But there are no clear sensible places to draw the lines. So Google uses what people used to call FUD by Microsoft (casting fear, uncertainty and doubt) to aid Google's interests.
Google has a tricky task to try and decide what links to give value to. Their decisions on how to treat certain kinds of influence seems wrong to me. But it seems to be something they can chose to do. Google also claims (I believe) that they don't bother to value links that would obviously provide value if those links are made nofollow by the websites (so say the links on Twitter that would obviously be useful to gage pagerank as it was discussed years ago).
Google seems to still claim that they are better off not using links that could be used by an algorithm to provide valuable insight because to do so would downplay their anti-spam efforts. It is a hard spot they are in. Until another search engine takes away a significant portion of their traffic I think they will stay on their current model. If wonderfulnewsearch.com grew to 8% of the market by December I bet Google would use nofollow links to judge merits of links next to immediately. It is a rich source of information that is being ignored. They decide the search result quality can sustain that dilution in order to put pressure on those that try to manipulate the results by playing to the algorithms.
It is a dynamic, evolving contest where Google tries to gain insight and others try to figure out how to take advantage of how Google gains insight to gain favor for themselves.
Google can't say how people should run their businesses. All Google says is if you link to sites in a way that we don't like we will then punish your site in our search results. Of course that clearly means that Google is willing to provide worse results to users in the case when the best results for you as a search user are from a site that did something that Google didn't like. As long as search users accept degrade search results Google can do this. If degraded search results caused users to flee Google they would stop this method of trying to influence sites.
But they have a tricky balance of trying to degrade the search results just enough to force sites into being fearful of having their traffic harmed but not so much that users of the search engine get tired of the degraded results and go elsewhere. The punishment portion is solely about the trading of worse search results to users today in order to get sites to follow the behavior Google wants to see.
The question of Google improving search results by devaluing links that are not valuable is a different, though related, question. That doesn't result in degraded search results. But that also has nothing to do with a site following Google's decision on where to draw the line of what is an influence that means you should mark your web links to tell Google that you don't value the page you are linking to. Deciding how much to value links is a tricky business. Google has a lot of money invested in doing that well. The decided to have guidelins and punishment in search results for how links are done. That seems to be Google's business. If you want them to be happy with you, you follow their guidelines. So for example if you want to get links you can do so in many ways, including many ways that are very similar to directly purchasing them but you can't do the one that is exactly directly purchasing them and not tell google that they shouldn't follow the link.
I am obviously critical of some of what Google is doing FUD :-) but I understand the difficult task they have. I can understand the way they are trying to tilt the terms of the "game" to help them provide search results that are useful. I am sure Google understands the points I make. They just look at the situation and make a few calls that are slightly different than I would make.
Comment posted on: Why did our PageRank go down?