If you can’t trust your police with it, then there is something fundamentally wrong with your society.
It's actually kind of terrible at that, because it's not perfectly accurate. Even if they could make it 99% accurate, the country has 300+ million people in it and for every person you told it to match, it'd give you three million false positives.
What it's used for is mass surveillance. You capture a face and there are millions of possible matches, but hey, one of the matches just bought something with a credit card 50 meters from there. Or they're carrying a cell phone billed to a person whose DMV photo matches that face. Then they put the cell phone down and travel on a vector from there past twelve more cameras.
You get everyone's location history and meatspace social graph in a database. That's way more useful to an oppressive regime than any kind of legitimate use, because you can get the same information from a suspect under investigation by conducting surveillance of only them, e.g. put a GPS on their car.
Retroactive surveillance can be used by future regimes to observe past behavior. Even if your existing government is trustworthy, there is an election every four years and turnkey fascism is inherently dangerous.
Meanwhile retroactivity not all that useful for legitimate purposes because criminals who are actively committing new crimes can be caught by non-retroactive surveillance specifically of them, whereas there is a much weaker state interest in interest in catching criminals who have already reformed and stopped committing new crimes.