Move fields on User edit page between pages
Neither are pretty but, you could customise SimpleNews so that it includes the correct form data on the page you want (instead of creating its own local menu entry), or use hook_form_alter() with a custom submit and validation handler on the user edit form to update the db properly - you should be able to steal the code from simplenews for the validation and submit, that way it will update the db the way it was intended. If you go for the former, new updates to simplenews will be difficult to deal with and if you go with the latter however, be aware that future updates to simplenews may break this functionality if they change table names, etc.