Because the intent isn't to terrorize or murder innocent people, but to hit "military" targets. It's akin to dropping bombs on military targets during WW II. Just because they are military targets doesn't mean innocent people aren't killed. So it's classified as something else. And before you go shooting the messenger, keep in mind your asking about classification. If you want to disagree and suggest that it's still murder, feel free to argue endlessly with others. But it's important to realize what you are then suggesting, and the ramifications.
War sucks. Innocent die.
Cue the pedantic replies.