Unrestricted immigration was the norm for a very long time and due process remained intact.

The Constitution says persons are owed due process, in two amendments. Due process isn't limited to only citizens.

I will point out it's a white American man, a rapist and a felon, and vile insurrectionist, along with his corrupt cadre, who are opposing due process. They are abducting people without charges in the night, scurrying them away to a different jurisdiction quickly, while actively slowing justice by inhibiting right to counsel. It's hallmark authoritarianism.