Because innocent people getting swept up by the legal system is potentially disastrous for those people, particularly if they are people of color. Take the Kalief Browder case as an example: http://www.newyorker.com/magazine/2014/10/06/before-the-law
However, injustices happen all over the world in varying degrees. It's important to figure out the balance of damage done / good done. If one out of 100 million people is wrongly convicted each year due to these practices, but 1 out of 50 million is rightly convicted each year, then this tool net good (even though innocent people have convicted.) This is just a simple view though, it's up to society to balance this equation (but that means not having a puritist and irrationally stubborn point).
What is the prevalence of accidental conviction due to the police having access to your limited private information?
