You are incorrect, no one wants that. However, victims of crimes and witnesses are less likely to come forward if they fear contact with local police will result in their immigration status coming into question.
When the undocumented immigrant is a suspect yet not arrested it is not reasonable to ask local law enforcement to hold that suspect if ICE has does not have the proper hold request filed. That is the case in many situations, ICE wants the police to hold a suspect while they file the paperwork, requiring the Police to hold someone without charge or legal reason because of ICE's incompetence. If there is a valid hold request on an individual, even "sanctuary cities" will honor the hold.
Your misunderstanding of the facts are not surprising. The right wing propaganda machine, including conservative talk radio and Fox News has you so brainwashed and misinformed you will believe anything. First Moron, Eric Trump's recent tweet implied that Ellen DeGeneres is part of the "deep state," I wouldn't be surprised if conservative conspiracy theorists take up the mantle and start spreading that nonsense, and in a few days you'll be posting that she should be investigated.