Totally speculative, but I imagine it's because of how the events played out. And just to be clear, I have zero experience with law or law enforcement.

Sutton and Zabavsky witnessed Brown committing a crime (riding a scooter without a helmet). They are obligated to perform their duties and stop him from committing that crime. However, their department policy (which as I understand are the guidelines under when, where, and how police officers are allowed to enforce their authority) states they aren't allowed to pursue Brown for that crime alone.

The officers lied to superiors about the events leading up to the crash, and Sutton's attorney told the judge they were making a legal stop to determine if Brown was armed. I'm assuming (because it isn't stated anywhere) it was determined that Sutton and Zabavsky didn't have probable cause to stop Brown for that, and by pursing him they acted outside of their authority.

I imagine the prosecution could argue they were committing a crime when Brown was hit. Again, all speculation.