I have found a site that contained a quote that I agree with:
Search engine optimization is just a means to help distribute your message. Nothing more, nothing less. Calling search engine optimization unethical is similar to calling creating a website or printing a newspaper unethical.When I started in the Web Effectiveness role just over a year ago, the concept of white and black hat practices also became part of my consciousness. The Wikipedia article on white vs. black hat also mentions the existence of grey hat techniques, which implies that instead of a dichotomy, there in fact exists a continuum of optimizing measures, where web pages can be boosted in SERP (search engine result page) rankings. This leads me to believe that there is no such thing as pure white or black practices in SEO, only shades of grey.
Search engine ranking algorithms are not just multi-layered and complicated, but also ever changing. I would describe black hat practices as not so much ways to contravene the supposed rules that search engines apply when indexing web content, but rather as tactics that accelerate the evolution of ranking algorithms, due to their exploitation of the weaknesses in bot-driven (without human intervention) classification that occurs for the vast majority of web content. On the other hand, there is no guarantee that these algorithms wouldn't change, even if every single entity on the web only employed search engine compliant techniques.
So, search engines control result rankings (and thus dictate what hue of grey various practices might be), which means they wield a lot of power. Does might make right? Are all search engines "ethically" generating their results? An interesting example was the allegation that Bing copied bogus results from Google, and the ensuing debate (here's a Searchengineland article from February 2011 that links in turn to the various coverage of the refutations).