Exactly! It did not do anything. You can't outlaw every object (rocks, sticks, bottles, etc) that could be used as a weapon because the objects do not cause the harm, it is the operator that causes the harm.
Maybe somebody should suggest to Piers that murder be outlawed, as that will surely fix the problem.
All crooks start out as law-abiding citizens until they break the law by doing something like murdering seventeen people. The law against murder did not stop the perpetrator, what makes Piers think that other laws would?
What actually could have stopped the perpetrator was the defensive use of force by one of the victims.