"Hit him back"

Can someone explain this logic to me? I see it as advice on here often. It really bothers me when the child is under two years old. There has been so much research that at this age it is impossible for a child to connect their action with this reaction. This is not about spanking older children. I won't spank my child, but I have to believe it is a personal and well researched choice by other parents who use this form of discipline. This is strictly about children under the age of two being hit or spanked as a form of corrective action.