Most of these departments have rules about how they use our data. ICE now gobbles it all up and can use it without rules by a department that operates with little regard and lots of exceptions to typical protections for citizens afforded by the constitution.
The majority in SCOTUS does not seem to care (it’s ok as long as their guy does it). Whatever rules we thought there were seem to be out the window because someone magically moved data or ICE got to do it or so on ...
Almost the entire US constitution applies to non-citizens in the country, with some small exceptions like voting and holding public office.
Without it, the executive gets to just say "that person shouldn't be here" and they can send them wherever the whims of the government are in the day.
Due process is how someone says "Hey government, you've made a mistake".
It isn't just due process. It's "I'm a US citizen, you can't legally deport me" Due process is what enables making that argument at all.