The ACLU has lobbied heavily for cities and counties to turn down detainer requests by ICE because they do not constitute an arrest warrant. A federal appeals court in 2014 ruled that detainers are merely requests rather than orders to local law enforcement, essentially making it optional for a local agency to honor them.
That's why you have to get them out and keep them out, on a rapid deport fast-track system, efficiently, quickly, and quietly.