Search engines like to paint SEO services which manipulate their relevancy algorithms as being unethical. Any particular technique is generally not typically associated with ethics, but is either effective or ineffective.
Some search marketers lacking in creativity tend to describe services sold by others as being unethical while their own services are ethical. Any particular technique is generally not typically associated with ethics, but is either effective or ineffective.
The only ethics issues associated with SEO are generally business ethics related issues. Two of the bigger frauds are
- Not disclosing risks: Some SEOs may use high risk techniques when they are not needed. Some may make that situation even worse by not disclosing potential risks to clients.
- Taking money & doing nothing: Since selling SEO services has almost no start up costs many of the people selling services may not actually know how to competently provide them. Some shady people claim to be SEOs and bilk money out of unsuspecting small businesses.
As long as the client is aware of potential risks there is nothing unethical about being aggressive.