Why did Court uphold LAPD ban asking immigration status?

My only question is someone they stop has no id and the police suspect he/she could be illegal immigrant from this court ruling will the police be required to write down whatever information is given.Like an illegal is wanted for murder may he offer the police an ID and since the police are not allowed to ask about about his immigration status and his id comes back clear would many criminal be unarrestable due to do not ask or do not tell policy and would this allow allowed illegals whom commit crimes without any fear of the police ? And does this same policy work at our prsions the do not ask- do not tell and is the illegal than allowed to be set free in America ? Would the border patrol also be subject to the same laws ?Just what laws apply to illegals ?