What procedural guarantees are permanent residents guaranteed under US law, that these people were not granted when they were denied entry, ie. what is 'due process' in the context of entry for permanent residents?
That question matters if you want to gauge the chances for a successful appeal in court.
Whether completely upending the lives of longterm residents without any prior warning is something that should be allowed to happen in a country is at least in part a moral question, and surely the ACLU is well within it's rights to answer that in the negative; nor, I dare say, would that answer seem very foreign to the average decent person.
That's the easiest, inarguable example.