In general, preventive detention is a practice that would be good because it would help to prevent crime, but bad because it would infringe on the basic right that people have to liberty unless they are convicted of a crime through the due process of the law.
Preventive detention is the detention of a person so as to prevent crime and/or to maintain order. A person who is held in preventive detention is not being held because of what they have already done. Instead, they are being held to prevent them from doing something that they might do in the future.
It is clear that preventive detention would be a good way of preventing and controlling crime. If we locked up the people who were most likely to commit crimes, there would be much less crime because those people would not have the ability to commit crimes (at least not crimes against the general public). We already use preventive detention on some people, most notably sex offenders who are deemed high risks of reoffending. We believe that it is worth it to do this because the benefit to society (protecting people from the offenders) outweighs the cost to the offenders’ rights.
However, allowing more expansive preventive detention programs would infringe on our basic right to due process. The 5th and 14th Amendments guarantee that we cannot be deprived of our liberty without the due process of law. This means that we cannot generally be imprisoned unless we are convicted of a crime. Imprisoning people for what they might do in the future would seem to go against the spirit of this guarantee.
This is a case in which there is a tradeoff. We could expand preventive detention if we wanted to increase our ability to control crime, but we would pay for it by losing some of the liberty that we prize so dearly.