That particular foul is enforced so inconsistently. Basically, how the fighter reacts often dictates how the ref enforces it.
Guys hit opponents on the ground all the time, and unfortunately, it usually gets a warning at best. When the fighter gets hurt or acts like he’s hurt, then the guy might get points deducted or even DQ’d.
Lennox Lewis hit Mike Acey 3 times when he was down and only got a warning. Mike Tyson hit Don Halpin and Frank Bruno on the ground and just got warnings. Happens all the time and it getting enforced is rare.
The all-time king of hitting guys on the ground is Terry Norris.