Constructing it properly would allow the feds to crack down on the city fathers and PDs that fail to go along with the plan.
"laws already on the books, and empower local police to enfore immigration law, not ignore it?"
For the same reason I would never turn in a hard working person I discovered had the misfortune to be classed as an illegal, there are others doing the same. This allows for a crackdown on the real problem characters.