Help me out....
How does the DA get to decide if the president orders all illegals not be prosecuted?
Whoops, I wrote that wrong. In practice the district attorneys decide but the call in the end is the president's as the DAs work for him.
We are running into a state vs state YMMV thing here. Things don't quite work that way around here. The police decide if they are going to arrest someone and charge them. The prosectuor decides if he should continue the charges. The judge (or jury) decides the person's guilt and their sentence. The governor has the power to grant pardons, change sentences, etc.
My point is that one person making that decision for a whole slew of people isn't the way our legal system works. Our system allows for discretion. It doesn't allow for blanket immunity for millions of people because...because someone said so? Case by case is one thing and it needs to be in place. Cases by the millions are what legislation is there for.
The police do the arrests, but the DA does the charging.
It's true that at any step in the process someone could decide not to do their part, which would likely end up with the person going free. The police could not arrest the guy to begin with, the DA could decide not to charge them, the judge/jury could decide not to convict, etc.
The thing is that police are a civil service appointment. If they don't do the arresting the commissioner wants they can get fired. The DA (in most cases) is an elected position, just like the president. If people don't like the job they are doing they can lose re-election, but other than that they have pretty wide discretion as to how they do their job.
I agree that legislation is the right way to do this, but that's not how things are going at the moment, unfortunately. Although the 5th circuit ruled against him, I find it likely that in the end Obama will win on using prosecutorial discretion here.