In my mind the difference here is that he was alone, committed his crime in a singular act, and now he's effectively out of commission. There's no terrorist at large to further perpetuate the terror. There's no looming sense of threat from this guy personally, and since he wasn't part of an organization, there's no nebulous "other" threat from that, either.
Had he enacted a long term plan where he was shooting at people randomly for days and making demands or something like that, then that's different in my eyes. At that point it becomes about using the fear of further violence as a tool and that's when you cross into terrorist land.
But who knows. I think DHS decided it'd be called a criminal act for now - so I guess that's what it gets to be called. Perhaps that label was politically motivated, but I can certainly understand the decision. Everything need not be a conspiracy and every large scale crime need not be a terrorist attack.