The goal of the sheriff and the prosecutor is to stand tall in their ongoing conflict with the state institution, of which this pentest is just the latest event

The goal of the sheriff and the prosecutor is to stand tall in their ongoing conflict with the state institution, of which this pentest is just the latest event. It's empathically not "we contracted with these people to do security work for us", it's that "our enemies from the Iowa state contracted these people to try and screw us". They succeeded in getting some of these enemies caught, so they will make a point to their local voters as "we sure showed those bastards from the state agencies that they have no right to come here and shove their noses in our business" (which they've already explicitly stated to local press in pretty much the same words), that's a win already for a certain class of the local voters. Their goals explictly include making it harder for 'enemy agents' to operate. If they use every technicality within the bounds of the law to make it as prolonged and difficult as possible, they're making a statement that attempting to attack them in this manner will result in retailiation - if it makes it hard for Iowa state to employ other pentesters in the future, excellent, mission accomplished, that fits the goals of these local agencies very well. They treat this mission as an explicit attack against them, and they want to deter attacks like these with whatever power they've got - which includes the ability to arrest people.