I'd say ads are harmful as well, don't see people fighting those this much.

“People does not fight harmful ads for now“ does not mean coinhive should be added to web sites without users’ consensus.

And It’s more harmful than ads as users know explicitly whether there are ads on web sites and have a choice to not visit them anymore. But silently mining without warning users of that is worse.

But ads are okay to add without users' consensus? Not really.

I wouldn't say mining was explicitly without warning, computer fans usually spun up.

You're confusing morality with legality. The morality regarding coinhive is a controversial topic, but its legality should still be defined under the rule of law. The Japanese police is acting on dubious legal grounds in arresting coinhive users given the lack of legal precedence and the vague wording of the law. Regardless of our opinions on coinhive, it is still worrying to see the police make arrests for seemingly arbitrary reasons.

That really depends; if it's communicated to the user that, by having the website open, they are mining cryptocurrency for the website owner as a sort of "payment" for being able to use the site, then that's totally ok. If they weren't informed, then it's still just a minor issue, at least compared to large-scale tracking of users that many other sites do.

The police started acting long after the website owner removed Coinhive from his website.