It’s because they have nothing else to try and criticize. No one can claim that Kizaru turning on his own men and giving the Strawhats an unbelievably easy escape would be better for the story. That would just be ludicrous!
It just galls them that Kizaru is more effective as a reluctant antagonist than some pirate villains who had no such qualms but couldn’t accomplish as much.
They’d rather have Kizaru be an idiot that wastes time with Luffy rather than fulfil the top priority he came for. Seems like some people just got too comfortable with incompetent villains from the previous arc.