but still doesn't give them any choice in the matter.
why should they? they announce it the users and that's enough. On sites with usual ads you are also not given the choice to turn them off. that's what adblock is for.
http://www.guiabitcoin.com/ <-- spanish blog, they use the JSMiner without telling the user
They should because they are using the user's electricity. They are taking money out of the website viewer's pocket every second that the user is on the site. A lot of less computer savvy users will not realize that this is what is happening, and therefore this is an unethical practice.