And just because someone doesn't have a criminal history doesn't immediately exclude them from prosecution. That's where facts and evidence come in.
I mean, come on. There were NINE dead bodies in a parking lot of a restaurant! And if you were part of a gang that contributed to those deaths, even if it ends up that you weren't directly involved in violence, you need to be investigated. And bail needs to be set in a way to discourage you jumping it. These guys are known to be transient. Many of them, anyway.
You must have misread something. Two thirds (66.7%) of those arrested have NO criminal records.