“Roberts did write that “neither respondents nor my colleagues have been able to identify any relevant, judicially manageable limits on the Secretary’s decision to put a core demographic question back on the census.” “
In other words the law does not mandate any prohibition on any demographic related question, and why any administration includes or excludes a demographic question changes not their ability to do so; but we here at the temple of law don’t like this administration, so even though what they did was legal, we will make trouble for them.
Almost. The fact is there's a law, the Administrative Procedures Act, that says even though an agency may have a right to do something the decision to do so can't be arbitrary and capricious. In other words, they have to state a good reason for making the decision.
The problem here is that Wilbur Ross and Commerce botched the justification to the point that the majority of the justices had to say it was just a pretext.
Commerce gets a do-over but they have to come up with a better justification.