@baris So the _key is as you described, the value is the user id, and the score is the watch state?
That's correct, and the records do not exist until a user explicitly changes the setting on the category page. Until they change it the default from the ACP is used.
Turn Hide user list and data from guests on in /admin/settings/user and hit save. You should also remove find/read privileges from guests if you don't want them to see content.