"No direct Harm Vs. Harm" is essentially "No Harm Vs. Harm". The "direct" is only included to limit it's interpretations or to be more specific. Proving "direct harm", in this case, is proving "harm". Without being able to prove a direct causal relationship, the government would have no justification to act.
Inaccurately and purposefully yelling "fire!" in a crowded space is a direct attempt to cause panic. In other words, you would never be prosecuted for yelling "fire!" in a crowded space when there actually was a fire. It is not the act of yelling "fire!" that is illegal, it is purposefully causing a false panic that is illegal.
