Rubbish. They don't want to. The tractors have license plates, allowing them to be pursued after the fact, and the NL police have firearms if they want to get spicy.

There have been a number of attacks in Europe of the form "deranged person drives vehicle into crowd". I'm sure the police wouldn't sit back and shrug if that was done with a tractor. The vehicle doesn't have magic properties, the driver has police protection.