Seting a BP sales emnployee based on the users is not standard system behaviour at any SAP version I have tested.
Check if there is a formatted search on the BP sales employee field that may be defaulting this, or check the business process used to create Business Partners - e.g. if they are using "Duplicate" the existing sales Employee will copy across.
Are you running any other add ons that mught be causing this (B1UP for example)?