Overstaying a visa, which is the most common way to be "illegal" is literally not a crime and is not handled by criminal courts in the US. It is strictly a civ

Overstaying a visa, which is the most common way to be "illegal" is literally not a crime and is not handled by criminal courts in the US. It is strictly a civil matter handled by civil courts (which can't impose jail time, by definition.) Sneaking into the country is a federal crime (a misdemeanor for the first offense and a felony for the second) and is handled by criminal courts.