Is it legally now?
Well, it never has been illegal to name yourself whatever you like. In that sense we have been welcoming to everyone since 6 to 8 years.
Why they are not renamed?
Because nobody took offense? And because we allow a lot in display names and your about me. There are usernames that insult the Chinese government. Or call for action due to certain recent events.
Resetting display names becomes a slippery slope quickly. For example: Who is to decide what is really offensive, slur, a subtle put down? What if the political situation changes and Brexit forces us to reset all names that refer to cities in the UK? You will then go by the handle user315642.
Flag a post of your own to let a mod know a users displayname might be offensive and let them handle it. Don't be disappointed if the name doesn't get reset.
I would personally find it very funny to reply to an F* user with
@Fuck You got that totally wrong!
but not all users appreciate my sense of humor, nor does the CoC leave much room for jokes.