Because not every person who has a gang tattoo is a murderer. In real life all sorts of people get forced into gangs as teenagers, do as little as possible and then leave or at least go nonactive as soon as they grow up. There are LOTS of stories of people in El Salvador being forced to join gangs even if they didn't want to. Sometimes they did it because they were afraid for their own life, sometimes they did it because they thought it would protect their family. Using, "Does he have a tattoo?" as the justification to lock someone up without trial or release date is fukking insane. I've known numerous former gang members who were amazing people, and many others who were just regular people. If you live in LA there are literally thousands of stories of gangbangers turning their lives around after a bad childhood. But, I'm sure without the slightest evidence you're going to say, "That's different there, I just know it is because I want to believe it is."
How many of those imprisoned tattooed people do you think have ever taken a life?