Why is so much effort expended to disprove terrorist motives, i.e., Ft. Hood massacre and now the UC Merced stabbings?

THE CALIFORNIA COLLEGE STUDENT who stabbed four people at UC Merced carried an image of the black flag of ISIS according to a report, as well as a handwritten manifesto with instructions to behead a student and multiple reminders to pray to Allah, yet authorities continue to insist Faisal Mohammad’s motives had nothing to do with radical Islam.
Dunleavy likened it to the case of Alton Nolen, who allegedly beheaded a former co-worker at a food plant in Oklahoma City in 2014. While he was angry over a workplace issue, his actions were likely the result of radicalization, said Dunleavy.

"ISIS is not looking for a few good men; one crazy will do," Dunleavy said. "This is what a lone wolf looks like."

The printout of the ISIS flag was reported Tuesday by the Merced Sun-Star, citing a law enforcement official who spoke on condition of anonymity on Monday. The official also said Mohammad’s manner of dress during the attack and some of the websites he may have visited in the days leading up to the incident ultimately led local authorities to hand control of the investigation over to the FBI. Warnke confirmed that decision, saying “new information” had been discovered on Saturday about Mohammad, who was shot and killed by police during the episode.
http://www.foxnews.com/us/2015/11/10/uc-merced-stabber-had-isis-flag-among-his-belongings-report-says/?intcmp=hpbt1