Sorry, Autosort does not apply to cases created via public case submission, but only to emails sent in. Cases created via public case submission allow the public user to specify the area they are filing a case for. Since Autosort primarily chooses an area, the functionality is overridden by the user choosing an area in public case submission.
Public case submission works best when it's behind a firewall (ironically) so that non-FogBugz users in your company can enter cases. If you are getting spammed in your public case submission form, we recommend switching over to a sendmail form that sends an email to your FogBugz mailbox.