Giving human food (table scraps, left overs, etc., ..) isn't at all bad as long as its not rotten. Just remember every dog is different as far as their tolerance to a diet change. Example: when changing pet food its recommend to do it by mixing both ...
If folks only knew themselves what pet foods consist of and its nutritional values compared to others (post manufacturing in kibble state) - they might laugh at themselves in private but, refuse to openly admit their ignorance.