I would imagine that if a man decided to drive off a cliff to kill himself, he would utter the same phrase, if he were Muslim.
I doubt the phrase is exclusive to terrorist scum.
If this was what he yelled it was possibly his motivation and we deserve to know. That does not mean I think all muslims are terrorists if that is what you insinuate.
If it was a confused white kid with Aryan nation tattoos who gunned down people based on their race would that motive not be mentioned in news coverage (and be investigated?).
Why the apparent double standard? Motive is reported all the time regarding crime. Columbine was investigated and discussed for weeks and I would argue that helped everyone (schools, law enforcement, and communities) be more conscious of the possibility.