The intention was to model the fact that a morphism in a category is
also a morphism in any super category, via the forgetful functor. With
the example above, if A and B are rings, then a ring morphism phi:
A->B is also a set morphism. However, at the level of parents, and as
noted in the documentation of sage.category.HomCategory?, this is
mathematically plain wrong: if A and B are rings, then Hom(A,B) in the
category of rings does not coincide with Hom(A,B) in the category of
sets.

I don't see why this should be wrong: Any ring homomorphism is a set homomorphism. Hence, `Hom_Rings()(A,B)`

is a subset of `Hom_Sets()(A,B)`

-- nobody claims that they coincide.