If Friedman's explanation is wrong, and "economics" suggests nothing of the sort, then why do or should criminal trials have a high standard of proof than civil trials?
Let's first make a semantic point. In criminal trials, the defendant, if convicted, is punished. In civil trials, there are no punishments. Some sort of just distribution of goods is enforced, and the loser may have to pay compensation or damages, but no "punishment" is administered. To the extent that "punitive" damages are assessed, there is already a conflation of criminal and civil law.
Now then, who punishes in a criminal trial? Society does or the state. Society harms the criminal for the sake of some benefit to itself (retribution or deterrence).
Here is the major difference. In a civil case, one man, Jones, sues, brings charges against another man, Smith. The battle is approximately equal; it's a mano-a-mano fight. Smith's resources to defend himself are more or less similar to Jones' resources to accuse and push his case against Smith. Moreover, a counter-suit is always possible.
It's a rare case when a rich man sues a poor one. What would be the point? More often, the accuser is poor and wants to stick his hands into someone's deep pockets. The rich guy then is usually on the defensive; so, even if the poor person loses, he does not become worse off than before; at worst for him, no money changes hands.
"Preponderance of evidence" simply means that whoever has the better case, "51%" or more "right" on his side, wins; just as in a boxing match: whoever scores more points is declared victor.
But in a criminal trial, the accuser, the state or the "people," is vastly more powerful than the individual. It has virtually unlimited resources: the tax money, cops, prosecutors, investigators, forensic analysts, the whole machine of justice, while the accused, especially if poor, may have to be content with a barely competent public defender.
In addition, the state has a lot of time to waste on the case; prosecuting people is its occupation lovingly pursued; whereas the accused has all the duties of a private citizen -- work, family, etc. -- to fulfill and so can defend himself only on a part-time basis.
It is the recognition that the state can easily crush the individual in a trial, засудить him, that has led to adding this extra limitation on government, the higher standard of proof. The citizen is protected against overzealous persecution on the part of society; it is yet another step on the road of defending the freedom of the individual against the power of the state, of de-communizing society. The latter is explicitly handicapped, so that the trial is a more equalized battle. The higher standard is simply a plank of classical liberalism.
(A specific further problem with our -- and every other -- government is that judges are employees of the state and therefore cannot help but favor the organization that pays them their salary. I have stated in several places that the judicial system should have been entirely private -- one of my concessions to "anarchism" -- from the start.)
Another such plank is the institution of grand jury. The public prosecutor cannot of his own volition even accuse a man of an infamous crime. He needs an assembly of the people to consent to do so. Unlike civil law where anyone can sue anyone else.
Finally, perhaps some people may feel that it is a greater miscarriage of justice to unjustly condemn an innocent man than to unjustly let five criminals walk. But then this would be a suggestion not, contra Friedman, of economics but of ethics or rather of some ethicists.