On the Internet, connections are king. If you web page is linked to by many other web pages, you can be considered an authority on one or more subjects. Some web site owners take this to an extreme and pay other web sites to link to their web site pages. Search engines frown up on this practice, because it interferes with an unbiased set of search results, which is what they are looking for. But web site owners often take advantage of this business proposition, because more traffic means more money, which offsets the risk of getting caught by the search engines. But is paying for links ethical
Paying for links is a form of marketing, and is just as ethical as every other form of marketing. Just because some people don't particularly care for it doesn't make it wrong. Sometimes you just need to get the word out about a new product or service, and this is the best possible way to go about it. Also, there is the theory of the tipping point. Any web site must reach a certain amount of users before it can start to grow on its own, spontaneously. Pushing the web site to that threshold of users can be extremely difficult, and a web site can actually die without even reaching that threshold. Making sure that the web site survives to become self-sustaining is important to any business, so if buying a few links is what it takes, then that is what the web site owner should do.
Besides, people have no obligation to visit a site if they come across a link or a search engine result. Nobody is holding a gun to their head to make them visit. They have completely free will and free clickability. There's nothing wrong with placing a link in their path, it is their choice to click on it or not.
People rely on search engines to give accurate results, and search engines rely on automated processes to ensure that they have accurate results. No automated process is fool-proof and fail-safe, but as long as they can deal with predictable variables they can offer the best service possible. If web site users intentionally try to influence the automated system to give themselves an unfair advantage of their competition, however, the automated system becomes useless to everybody. Because Google has become such an important aspect of everyone's life, interfering with its search results is like interfering with morning commute traffic patterns. It is not usually life-threatening, not usually illegal or evil, but it is quite annoying to a great many people. If a person wants to live an ethical life they should try to avoid annoying large amounts of people for personal gain.
Also, web site owners have an obligation to Google for giving them visitors in the first place. Without an indexed Internet, people have to fall back onto traditional methods of marketing in order to get information about new web sites. Nobody wants that. People who benefit from Google's service have an ethical obligation to use it honorably, and not to twist the system for their own personal benefit.