It could reflect either fewer offences or laxer discipline.
In the former case, you'd expect the sentences to remain severe for the offences that still surface, but how would you explain the ludicrously low sentences handed out for the My Lai massacre and the few war crimes and atrocities that do come to trial from Iraq? Don't they indicate that it is laxer discipline rather than fewer offences that accounts for the lack of military executions?
Actually, as the military is generally a reflection of society (and surprisingly progressive in many areas) the lower penalties are a reflection of the lower penalties for all crimes throughout US society. Rape, for example, used to be a capital offense (it still is, in the military - but that is virtually never enforced). But even where execution might be appropriate that extreme penalty is seldom applied (except in Texas!). There is a misconception that military courts differ from civil courts. In fact, they are basically the same. It is absolutely true that a military court is likely to have a much higher CONVICTION rate, but the punishments meted out tend to be about the same as civilian courts.
One reason the conviction rate is so much higher in military courts is that there are so many safeguards and lesser tools to use before going into a courtroom. Every soldier has the right to free legal counsel (I spelled it right that time, damn it!) and so is less likely to be railroaded. And many offenses get handled by the chain of command using nonjudicial punishment (the ubiquitous Article XV) or adminstrative means instead of going before a judge. So by the time all of the safeguards and alternatives have been exhausted, anything that gets to a courtroom is likely to have a high likelihood of conviction.
It's funny you chose the word discipline, MT, because when a soldier sees that word it has a much broader definition than to a civilian. When I first saw your post I thought "Wow, that's an interesting point coming from MT." I was reading it from a military POV. But you are using it in a context that is more synonymous with "punishment' than what a soldier understands as discipline. When we talk about poor discipline, we don't mean lighter punishment, but rather lower standards of self-discipline. Well disciplined troops, for example, will perform instinctively correctly in a battle. They will react instantly when given a command. They will keep their are policed, their equipment in readiness and their physical condition to a high state. Such well-disciplined soldiers perform as teams, and almost always prevail over lesser disciplined forces. When poorly disciplined armies are stressed, like the Union at First Bull Run, discipline fails and chaos ensues. Of course, stricter punishments can add to that sense of discipline - though it can also detract - but the most important factor in maintaining discipline is training. Not quibbling about your word usage, I just thought it was interesting.