It's less about what they did after arriving and more about border violation itself.
Immigration has an official legal process to follow.
Why is that so bad? If the law needs to change - fine, change it. But allowing people to break the law is to enable collapse of the rule of law.
Ah, the law. Did the Stasi break the law?
Forgive my comparison, but it illustrates the problem.
The law cannot be allowed to become lawless itself, depriving the people of their rights to fast track the pre-determined punishment.