Pardon me, but that's an unworkable standard. Acts such as murder, rape, robbery, embezzlement are all against the law, yet people still commit those crimes regardless -- does that mean we shouldn't have laws making those acts illegal in the first place?

No, I tend to agree with B on this - none of those comparisons are seen as constitutional rights. Any attempt to interfere with such rights must at least have some prospect of achieving the desired objective although I'd be inclined to replace his use of the verb 'would' with 'could' (have prevented the incident).