Like I said, when an AD champion would otherwise be too strong with abilities, they have useless AP scaling to balance them out.
That would be true if all AD scaling champions had AP scaling. I can point to plenty that don't. No AP scaling on Zed, Talon, etc, because using AP ratios to balance champions isn't actually what Riot does
AP scaling isn't used to balance champions out. That's never the point of it. It's used as a design choice, not a balance choice. It is used when a skill has a reason to not need numbers to be useful. For Wukong the decoy functions without doing dmaage, so giving it AP scaling just ensures that it won't scale in terms of damage. If he needed it to do damage, it would have AD scaling.
Riven's shield needs to have numbers to work. Shields have no value if they have no way to scale. It needs to have AD scaling, or at least HP scaling, if it is going to be a useful mechanic on her kit.
That's why Riot has moved away from purely AP scaling shields on AD or Tanky champions. It's why they changed Sion's shield to max HP, Skarner's shield to max HP, and Urgot's shield to mana. AP scaling shields just become useless as the game moves on, and it makes the mechanic pointless as the game goes on. AP scaling shields on AD or tanky are not a design decisin Riot has chosen to support.