Let's say someone writes a blog article. They publish it on Wordpress with a bunch of unoptimised defaults. Some time after, someone comes along and says "Hey, the search engine will rank your post more highly if you include the name of the post in the URL instead of just having it be blog.example.com/posts?id=1234", so they change the URL. Is that SEO? Should that result in the site being de-listed/banned? It doesn't meet your definition of "provide website content that is more relevant/interesting for the viewer".
And where is the line between "provide website content that is more relevant/interesting for the viewer" and SEO? If I realise that (to take an example from something found on Google) I could take the sentence "Identify the best customers and convert more" and rewrite it as "Marketing automation helps you identify the best customers and convert more", is that SEO because I'm intentionally adding a keyword, or is that making the content more relevant to the user by being more explicit in what I'm saying?
I'm sure there are better examples, but this specific one actually does make the experience better for the reader. People often return to good blog posts and it's a lot easier to remember a title than a numerical id. If you can remember the title and know the url of the blog and it has a simple naming convention, you can bypass the search engine entirely and just go straight there. Sports sites are really good about this. <URL>/<LEAGUE>/stats always gets you the stats. <URL>/<LEAGUE>/standings gets you the standings. If you're curious how a team is doing or who currently leads in scoring or whatever, you know where to go without needing assistance from search.
To my moral standard, all of this is manipulative if the intention is to manipulate the Google ranking instead of seeing an improved Google ranking as a result of delivering better service to the visitor.
This does of course not imply that these things can be proven, which implies that no action can be taken. Not everything that is immoral is punishable.
And where is the line between "provide website content that is more relevant/interesting for the viewer" and SEO? If I realise that (to take an example from something found on Google) I could take the sentence "Identify the best customers and convert more" and rewrite it as "Marketing automation helps you identify the best customers and convert more", is that SEO because I'm intentionally adding a keyword, or is that making the content more relevant to the user by being more explicit in what I'm saying?