When a keeper goes to kick the ball out, the ball will only be in one hand as he winds up for it. Without that rule basically allowing an attacker to stand and flick the ball out of his hands everytime he tries to kick it out
Sure you could argue that even with the rule, the ball is out of the keeper's hands right before he kicks it, so what's to stop an attacker blocking it even with that rule? That's where sportsmanship comes into it, but I think the keeper should have to have 2 hands on the ball without attackers being able to make contact with it. IMO, the keeper was careless with it and deserved to be punished. If that was the Liverpool keeper and it was disallowed, I'd feel incredibly relieved and lucky.