I think bombers broadcasting warning messages is somewhat more common with certain types of political violence. I know weather underground bombings tried to choose times for their bombings when buildings would not be inhabited, not always successfully.
If a group is hoping for a negotiated settlement they basically have to do it that way, both because mass casualty bombings tend to make negotiations impossible and because it increases confidence that a group could stop the bombings if their demands are met. Deadly bombings only seem to work when the group using them is trying to start a war.
