Ethics varies with the context, and often has a personal and cultural baggage. So what is ethical to you may be unethical for someone else under different circumstances.
Let's take the example you have given and consider it from the perspective of a corporate environment.
Suppose I wanted to promote someone from my team. But my boss doesn't like them for whatever reason and asks to consider outside candidates too for the post. I feel I have an ethical obligation to help my junior team mate advance professionally. And selfishly, I'd also rather work with someone whose work and work ethics I am familiar with and trust. So I'd definitely prep him / her for the upcoming interview to boost their chances.
Obviously, this will place many outside candidates at a disadvantage.
But from my perspective, I feel have no obligations (ethical or otherwise) towards these outside candidates I don't know or care about.
So what about an ethical obligation towards the company to hire the most talented? Well, if the company doesn't trust my judgement to promote someone from within, then it essentially comes down to work politics - are my boss right and what is in my best interest? And If I believe that promoting from within is the right step for both me and in the company's interest, I'd be quite willing to indulge in some work politics of my own to get this colleague the post.
In a corporate environment, ethical violations are only considered seriously if it is really serious and obvious - like someone lying, or undisclosed nepotism etc. And thus each complaint may have different result depending on the person who judges it. (Breaking the law is a different issue though).
Also understand that once you reach a managerial level, your further rise also depends a lot on the people you know, and not just your professional talent. This is because managing people requires social skills.