How can you indict somebody for a crime that they didn't commit? But they might do. I find that a baffling concept. How is it justified?
What is the legal status of predictive policing?
You mean this?
First, "predictive policing" is a very wide concept. When police send officers to a sporting event or a protest rally they are practicing "predictive policing". If we are specifically considering the use of algorithms in making the predictions then that is perfectly legal although there is a moral hazard in perpetuating unfair biases.
How can you indict somebody for a crime that they didn't commit?
Theoretically, you can't - indictments have to follow the offence and they are leveled at the alleged perpetrator of that offence.
Practically, it happens all the time - innocent people get indicted for crimes due to all sorts of innocent mistakes, inadvertent biases and sometimes outright maliciousness.
I don't see what this has to do with "predictive policing" however.
The determination of whether a person did a thing is made at trial, so actual guilt logically has to be follow the accusation. Whether or not that formal accusation is in the form of a grand jury indictment (as must be the case in US federal crimes, pursuant to the 5th Amendment), or by a prosecutor filing a charging document with the court, this has to be done before the accused is legally proven to have committed the act. There is a legal rule to the effect that if there is not probable cause to say that that the accused committed the alleged act, the charge must be dismissed. So not only do you have to have good-enough reason to think that the accused did the act, you also have to have good enough reason to think that the act too place.
A runaway grand jury could indict a person on the belief that he may commit a criminal act, but such an indictment would be dismissed in court because no actual crime had occurred.