I think the reason why African Americans push the one-drop rule on mixed race individuals because they want them to acknowledge it. American history has shown that bi-racials, tri-racials, etc. who have some African ancestry sometimes deny their African mix and don't claim it but are eager to claim their other racial mixture(s). So, it's kinda of a way to force them to acknowledge that their half, quarter, etc. black/African side like they acknowledge the other races/ethnicities their mixed with.