Do you guys think its OK (legally or morally) for a website owner to bruteforce the password hashes of a number of users of their website, and then use the resulting passwords to access their accounts on other websites and email addresses?
It is not OK, it is
dumb to bruteforce the password hashes of a number of users of their website, because before the password was hashed, the password was known to the website owner

Not if the password was hashed by the client. In this case the unhashed password is never available to the server.