Why do they need to be arrested?

After watching Minority Report the whole big moral argument seems to be that arresting the potential murders is wrong because they haven't committed a crime and it is possible they wouldn't. But we already know that the future can change. The precrime unit stops EVERY murder. But the movie seems to think physical force can stop crime but mental influences can't? The only people we have seen who have been able to change their future and not commit a murder are those who already know that the murder will occur.
But beyond this the ethical dilemma is dumb.
It seems to be:
Either arrest people who haven't actually committed a crime or allow people to commit murders.

The flaw is the human side of it. No murder is being committed. The system is perfect. The problem, the flaw is what is being done with the potential murderers.
 How about instead we simply stop the people from committing the murder (as we saw in the first scene where they stopped the man from stabbing his wife), put them in counselling or something, give them a fine (which would help keep precrime funded) and let them go on their way. The entire MORAL DILEMMA of the film is based mostly on the imprisonment of the criminals. and that imprisonment is totally unnecessary. The precrime unit has proven they are already capable of stopping the crime.

If you take out the imprisonment of the potential murderers there really isn't much of an ethical dilemma. The most that could be argued is for that of the 3 precogs. And then 3 peoples happiness to prevent all murders? This seems like a good trade

I do not like the moral choice being presented as only two options. There is more depth to it and the fact that the system prevented all murders shows how effective it actually is.