> These are all tasks for the state to do before arresting someone.
There were certainly done in the case of these 36. That isnt the issue. The issue is that there are some non-zero pool of US citizens, who associate with non-US citizens illegally in the country, who cannot readily prove their citizenship. Since they are present with illegal immigrants, and are reasonably suspected of being one, they are reasonably detained.
> There's almost no way it couldn't be improved.
This indicates you aren't aware of the issues involved.
No one thinks its a good thing for US citizens to be mistakenly detained. The onus is on people who agree that there should be some level of immigration enforcement to explain how a perfect process avoids 100% of all such cases.
If you dont want immigration enforcement, that's a clearer position. Then the question is: what does the US owe to non-citizens who have build nothing, and paid nothing? If the answer is "nothing" you've got a lot of homelesssness, medical, etc. issues to address --- if your answer is "something" then you've got 8bn people on the planet, and the question is: how much?