I'd agree with that part of the debate but how many ARV's do you need and where do you put them to ensure that they are where the atrocity happens - and even then it doesn't stop the first part of the attack (unless all vans drivers who drive badly are to be shot on sight - which might put me at significant risk!)
As far as prevention goes - yes more/better/more effective/better directed has got to be a better use of finite resources but, and it's a big but, what do you do when you suspect somebody of having the potential to commit murder but no laws have been broken?