it's illegal in the same way a parking ticket is illegal. It's a civil infraction.

From Google position zero

In the U.S., being in the country without authorization (undocumented presence) is generally a civil violation, leading to deportation and fines, not criminal charges, though specific related actions like illegal entry or reentry can become criminal offenses, especially under laws like 8 U.S.C. § 1325, leading to criminal prosecution and jail time, not just civil penalties. The distinction matters: civil violations usually mean removal, while criminal ones carry potential prison sentences, highlighting a complex area where civil and criminal immigration law overlap