I’m not trying to be contentious, but perhaps his policy is that which he perceives as the best way to enforce that which you see as the ‘prime directive’.
In other words, he can conceivably send in the “feds” to all 50 states and enforce the law- or- he can tell the states to deal with their immigration problem in compliance with federal law- or suffer the consequences.
It’s not an abdication of responsibility- it’s a method to fulfill it.
Maybe we should no longer pay Federal taxes or drastically less and pay the Border States a lot more.