Back when the movie was new, there was some talk around these parts about the roles of Superman and Richard in Jason's life. That's why Richard not only isn't a straw man, but is given a heroic role...he's being proven worthy of raising Superman's son. And I think that Superman accepts the situation because he knows that he could never offer his son the sort of normal upbringing that Lois and Richard can. That's why I'd wanted to see where things went from there in a hypothetical sequel...but that was never to be.
The first source that I know of that had only Pa Kent die was the George Reeves TV series...I think that a lot of things in the Donner film were informed by that source, so it got repeated there.
As portrayed in both origin films, though, there is a certain symmetry to it...Clark loses his Earth father only to discover his Kryptonian father.