Why would adultery be considered a crime in the 23rd century Federation when it wasn't considered a crime on 20th century United States, the
de facto role model of the UFP?
Dark Journey's point is quite relevant here: crime in the UFP is down because the UFP does something about it. Things would thus get worse the farther away one gets from whatever the UFP is doing.
What
is being done, then? We know that criminals get psychological adjustment for everything ranging from Harry Mudd's petty schemes involving counterfeit currency, false identity or dealing in illegal goods, to Tom Paris' or Kasidy Yates' aiding and abetting the enemy, to Garak's attempted xenocide. We know that incurables are so few in number that they all fit in the facility on Elba II - out of a probable population of trillions! We do not know of any repeat offenders: whenever a psych-adjusted former criminal commits a new crime, it's different from what he or she did before. Mudd never used counterfeit money again AFAWK, Yates never smuggled for the Maquis, Garak never considered killing all the Founders through sabotage and unlawful use of Starfleet equipment, and so forth. In contrast, Lon Suder kept on killing, but he never was subjected to a UFP anti-criminal reconditioning program.
That's the reactive part, and perhaps "crime is down" simply because it's swiftly dealt with in this manner. But there's also a preemptive element to it, apparently. From TNG "Justice":
Rivan, foil of the week: "We are a people of law. They do sometimes bring us sadness, but we have learned to adjust to that. Perhaps your laws work as well."
Picard: "They haven't always, but now they do."
Liator, another plot complication: "Do you execute criminals?"
Picard: "No, not any longer."
Rivan: "But you did once?"
Picard: "Unfortunately, yes. But since then-"
Rivan: "But when you did, was it believed necessary to do so?"
Picard: "Some people felt that it was necessary. But we have learned to detect the seeds of criminal behaviour. Capital punishment, in our world, is no longer considered a justifiable deterrent."
So it seems that some sort of a screening program allows for crime to be preempted without resorting to severe deterrent. We know that fines still exist as a form of deterrent, at least for some non-UFP citizens (Quark got fined by the Federation once), but we hear of no other deterrent such as a jail term. Imprisonment always seems to be solely for the purpose of forced reconditioning, not for any sort of punitive aspect.
OTOH, from an earlier part of the same episode:
Yar: "That is extraordinary!"
Worf: "Can I take a moment of your time, Lieutenant?"
Yar: "Yes, Worf, but you've got to hear this. Are you telling me that there's no crime here whatsoever? No one breaks any laws?"
Liator: "Once they did. Long, long ago there was much disorder. But not now."
Yar: "But I seen no sign of police. Those who enforce laws."
Rivan: "Oh, we have very few. They are called Mediators. And they are needed only in one place each day."
So Tasha Yar, a Federation citizen and Starfleet officer, expects that law is enforced by police, and thinks that total lack of crime is "extraordinary". Combined with the fact that our heroes in all the shows encountered UFP citizens engaged in criminal activity, it seems we have our answer.
Timo Saloniemi