“I know nothing about US deportation policy, even whether it’s a federal or state-by-state law.”
All matters regarding immigration are federal matters. However, many states will turn over suspected illegal aliens to the federal government for possible deportation.
It is actually quite rare for it to be a crime to be in the US illegally. Most of the time, it is a simple violation, like a parking ticket. Usually the federal government will allow an illegal alien who has been detained to leave on his/her own, at his/her own expense. This policy saves the US taxpayer a huge amount of money, and allows them to apply to enter the US legally at a later date. Generally, anyone who is actually deported is ineligible to even apply to come to the US for ten years, and permission for someone previously deported to come to the US legally is very rarely granted. About 40% of illegal aliens actually came to the US legally, but overstayed their visas. Furthermore, a huge fraction of the illegal aliens actually qualify to be in the US legally by doing something like enrolling in school or enlisting in the US military. President Obama has been savagely attacked by the nativist bigots by encouraging illegal immigrants to do this rather than getting deported; many of them were brought here as children by their parents, have little or no memory of their country of nationality and have close relatives who are US citizens.
Persons committed of serious crimes are generally deported after serving part or all of their prison terms. The US has agreements with many other countries where an illegal alien can serve part of their sentence in their country of nationality, again saving the US taxpayer a huge amount of money. Many countries release such persons fairly quickly, though.
And when persons who have been deported after having been convicted of a felony return to the US illegally, that is when being in the US illegally becomes a felony. One of the people we indicted had been convicted of first degree murder in Texas and had been deported back to his home country after serving about half of a 25 year sentence. He had returned to the US and I hope that he will be getting free room and board here for a long time. The Obama administration has been aggressively prosecution these previously-deported illegal aliens like no administration before. I was surprised by this when I served on the jury; I had assumed that corrupt politicians, stock swindlers, and drug lords would have been considered a higher priority, but 80% of our indictments were for illegal aliens who had previously been deported after having been convicted of a felony.