When you shop in a grocery, supermarket and purchase closed packages, you check the hashgocha and decide if that's your standard, When you go to a takeout, Pizza, Ice cream, Yogurt bar, etc you don't know what you are getting. It's common by a take out to at least ask who's meat & poultry do you use, even just sometimes?
It's incumbent on every kosher consumer to check up and ask questions on the source of the ingredients being used in Ice cream, Yogurt, topping, flavors, etc.
You might be surprised. A kosher certifier may allow items which you wouldn't use at home, even if it's cholov yisroel (who's?).