If a user is banned, their account is banned (their email address is part of their account details) A user can not register with an email address already used by an account (they also can not register with an existing username)
So on banning an account, if the user attempts to register with the same email address, they can't (because it is already used by a user, even if it is a banned user)
IP addresses are not considered part of the account details that can uniquely distinguish a 'user'
usernames/email_addresses/user_id/session_cookies do uniquely distinguish a 'user'
This is why email addresses do not manually need to be added, but if you wanted to ban the IP address, you would need to manually do it