It's amazing the assumptionf different folks make. These assumptions may never even be reflected in, or inferred to by, standards and codes....but they are there nonetheless.
Having dealt with some folks in the design and testing of receptacles....they, like most real electricians, assume that only a DIY will actually connect two set of wire to a device- surely not more than that! It is also recognised that it is a fairly common trade practice to "split" a receptacle. (Switched receptacles for lighting and the dishwasher/disposal receptacle are prime examples of this).
And, unless you're a HIY or tract hacker, you use a pigtail.
Another assumption these days is that you'll be using a "greenie" to join the grounds together.
Until recently, it was quite possible to legally wire a receptacle without connecting the ground wire directly to it; a connection to the metal box was considered sufficient. Even today, I think it is assumed that the primary connection will be to the box, with a eparate pigtail to the receptacle.
Receptacles probably would have been made completelt without a ground screw, except that, in plastic boxes, you really need to attach that wire to something! I'm certain this is the real reason switches have ground screws....speculation about metal cover plates notwithstanding!
