See the article here:
http://law.onecle.com/constitution/article-3/32-original-jurisdiction.html
I’m not a lawyer, but I know enough to realize what seems obvious at first glance is not always true under the law.
“Exercise of its original jurisdiction is not obligatory on the Court but discretionary, to be determined on a case-by-case basis on grounds of practical necessity.”
It seems that indeed, the SCOTUS has relinquished some of it’s power to the lower courts. Especially in Civil cases.
Very unfortunate for Arizona, or any other state that wants to pass similar laws.
However, there are other points that can be argued in the Appellate court, such as presumed ‘intent’ of the law.
Something the Federal government would need to prove, which at this point, would be a little hard to do since there currently are no ‘injured’ parties to the suit.