Ethical Link Building means you have to earn link, i.e credibility, reference and reputation. The core purpose of link is this not to just put your links any where. Also you should ensure that you are not involved link buy sell, exchange activities and other similar. When you talk about ethical then it must meant for value addition of end user not for your website.
SEO is meant for aligning objective of your website with its stakeholders, it is not just limited to SEO Guys who see it with the prospect of link building and keyword rich content, it is also about usability and giving optimum experience to searcher - I wish you all too see it from this glass.
Hope it helps!!