Even if this system works, people who have not committed a crime (yet) are innocent.
Taking an innocent person, and improsoning him against his will is a crime, pretty extreme one.
Even if this system works really well, it's hard to imagine that the crimes it prevents can be more extreme than the crimes it commits.
Every criminal justice system has to balance (1) the risk of convicting the innocent, (2) the risk of letting the true crime-committer commit additional crimes (due to not being caught or caught later), and (3) the risk of being abused. We don’t like pre-crime convictions because we believe risks 1 and 3 outweigh any gains in 2. A culture with a different balance of risks, however, could come to a different but consistent conclusion. I don’t like that conclusion. But it’s at least debatable. Putting political enemies in jail is not.