Why is the US scared to charge the Oklahoma beheader with terrorism?

He said he did it due to Islamic beliefs and learned from watching ISIS videos. He and his mom had terrorist pictures on their facebook (pics of twin towers, Bin Laden, ISIS, etc) and other shite on their pc's. He confessed to the crime and said he did it because of USA bombing in Syria/Iraq. Yet the FBI says they found "no indication that Alton Alexander Nolen was copying the beheadings of journalists in Syria by the Islamic State ... adding that they are treating this as an incident of workplace violence."

He also posted this on his FB:
"This do we find the clear precedent that explains the particular penchant of Islamic terrorists to behead their victims, it is merely another precedent bestowed by their Prophet." He also added a citation from the Quran: "I will instill terror into the hearts of the Unbelievers; smite ye about their necks." "Smite" means a heavy blow or stroke with a weapon. The terrorist who killed Colleen Hufford "smote" her neck when he cut it, reportedly with the same knife he used to slice produce when he worked at Vaughan Foods.