I agree with @maegul that this is probably just a poorly built exercise and/or overlooked by the creators on their “final” pass. Especially given that printing out the unit type () (as the code in main() effectively does) makes very little sense.

However, I do see a way to “fix” the ownership “issues” that makes the rest of main() make sense, without touching main() itself:

fn take_ownership(s: String) -> String {
    println!("{}", s);
    s
}

Basically what you stated you did to solve it, though I wanted to highlight that you might want/need to explicit the return type as well as actually returning the s parameter.