It's only "common sense" because at the quick/counter service restaurants I patronize at home customers throw out their own garbage.
Some parts of the country/world CS restaurants have employees that clear the table. Likewise guests who observe CMs clearing tables might get the wrong idea.
When Wendy's first opened they didn't have any garbage pails out for customer use. They had employees that cleared the tables.
I don't remember if I cleared my own table at the Pepper Market or if CMs bused the tables.
Some parts of the country/world CS restaurants have employees that clear the table. Likewise guests who observe CMs clearing tables might get the wrong idea.
When Wendy's first opened they didn't have any garbage pails out for customer use. They had employees that cleared the tables.
I don't remember if I cleared my own table at the Pepper Market or if CMs bused the tables.
Forgetting about posted signs with rules it should be common sense to throw out your food when your finished eating at a counter service location. Just because WDW has rules for what you mentioned and they don't have a rule about throwing garbage, maybe it's because they want to assume that all their guests would at least do that.