We're generally cautious about assuming all new accounts will want to be authors -- as you've seen, users tend not to read the forms very carefully, and a default checked "sign up as author" checkbox would result in many author accounts for users who don't actually want to be authos. This becomes relevant if you want to e.g. contact all users with an author role; you'll end up contacting many users for whom it's not relevant.
Users who don't have an author role but who follow the "submit" link anyway should be granted an author role automatically, as long as author registration is open (check your timeline dates).
If you want to change the defaults anyway, the code in question is in classes/user/form/CreateAccountForm.inc.php, particularly in the initData function.
Public Knowledge Project Team