The problem is that those of us/them who are outraged enough to continue the investigation are not the people in charge of prosecuting. If we/they just say "ah, no one will be charged, therefore we don't need to continue the investigation" every time something like this comes up, then one day we won't even have investigations. It is a slippery slope.
A swift, public prosecution of even the most minor malfeasance is the only true power we have over the elected official. Voting the jerk out of office can take years and doesn't really punish him, nor does it disuade other jerks from doing the same things.