Because it is. How would delete account work in a threaded forum? Delete all your posts?

Give you three options:

1. delete all your posts the same way they would be deleted if you went to them and pressed the delete button.

2. Anonymize them (replace user name with "deleted user <randomnumber>").

3. Keep storing the username

After that, delete all other data (profile, settings, e-mail, password hash, ...)

Yes.