What's the legal justification for striking down commissions?
A hyperliteral reading of Article I Section 4 so that only a State legislature may devise district maps for the House of Representatives.
But that ignores that State legislatures can delegate the drawing of Congressional districts to non-partisan commissions. What they cannot do is to draft districts that grossly disenfranchise large segments of the population. Michigan is a prime example: if you live more than ten miles west of US 23 or to the north of Bay City and you are a Democrat, your voice is unlikely to be heard in the House of Representatives. The Koch syndicate owns your Representative and pulls the strings.
Michigan is not heavily gerrymandered. Most of the Democratic areas are in and around Detroit. There are Democrats in Southwest MI but its still culturally conservative I would think thus that area of MI elects Republicans to Congress.
Its the eighth district that is their primary objection if I am not mistaken. Though the current map has other flaws as well and the process was clearly rigged, hency why they need a commission in my view.