Sanctuary implies a place of refuge for the oppressed. But the purpose of these cities is to provide safe haven for the criminal class -- that is, oppressors. So "sanctuary" is really the opposite of what they provide.
There is a perfectly good legal term for those who enable criminal behavior before it occurs, or provide cover for them after they commit their crimes, both of which these "Sanctuary Cities" do. It is called "accessory." So we need to start labeling these Accessory Cities to make clear what function it is they perform.
I prefer the term sedition or seditious as opposed to sanctuary