Key part of that article:
“We conducted a thorough investigation into this incident. However, we were not able to conclusively determine that the suspected drone operator was the pilot. That said, we did counsel the drone pilot, both orally and in writing, about the importance of not flying drones near wildfires and observing any [flight restrictions] that are in place.”
~FAA spokesperson Ian Gregor
An FAA investigation is pretty thorough. and unless they can definitively prove that pilot was the one piloting the drone, they can't proceed and pass it along to DOT IOG for prosecution. If the ASI decided they couldn't prove who the pilot was, they can't bump it up.