Because it lacked intent to intimindate or coerce a civilian population or to influence the policy of a government by intimidation or coercion.
That's the definition right out of the Patriot Act.
His actions were criminal, not terroristic. Remember, crimes are a *bad thing* too.
Intent. The man is an idiot who broke safety laws via some sort of twisted thrill seeking. I doubt he set out to harm anyone. When I was a kid, my brothers and I shined a great big light at a bunch of passing Hueys near Ft. Campbell, Kentucky. We were being cute. The Hueys turned around, landed in a field, and disgorged a platoon of troops ... who were also thrill-seeking. I learned an awful lot about escape and evasion that night, but nobody got hurt or was charged with any crime.
What people do MUST be judged by intent. This is the same malfunction that gets little kids charged with crimes when they carry a one-inch plastic GI-Joe "weapon" to school.