The usual standard for arson involves burning with some intent to damage whereas in this case the burning improved the public land. For that reason the Harney County Sheriff and District attorney declined to prosecute what would clearly have been misdemeanors at most under state law at the time of the crime. Poaching is also a misdemeanor but was never prosecuted in state court. Most crimes on federal property are adjudicated locally. 
The federal prosecutor rolled up 10 years of disputes with the Hammonds and took them to federal court but offered them a plea deal that involved no jail time. Of all the charges they were convicted of only the two fires they admitted to setting for reasons obviously not believed by the jury. Judge Hogan described the federal minimum for a crime clearly not in the same category as burning a building as cruel and unusual. Unprecedentedly Paganini appealed, won and dragged them back for the remainder 4+ years. This always struck me as vindictive.