Where are you guys getting your facts from? Did I miss something? The initial articles about this, before she was charged, say she was put on leave pending the investigation. After it was announced that they intended to charge her with manslaughter, I see nothing except the article linked by Vic says they (the press) didn't know if she was in custody or not. A Google search returned this from CNN just an hour ago which says the police are getting an arrest warrant for her:
https://www.cnn.com/2018/09/07/us/dallas-police-shooting/index.html
Are you saying they should have taken her into custody before investigating, before charging, and before getting an arrest warrant?
I can only assume here that once they arrest her, probably today unless she's on the run, she will either be in jail or out on bail, just like anyone else.