In Minnesota, the arrest is the punishment.
If they arrest, they don't have the capacity to prosecute. If they prosecute, they aren't going to convince a jury. If they can't stop legal observers with the threat of a brutal arrest, they have nothing.

Justice Department bracing for more resignations in Minnesota | CNN Politics
The Justice Department is bracing for a new wave of resignations in the Minnesota federal prosecutors’ office over recent immigration enforcement efforts in the state, sources familiar with the matter...