The decision by Bahrain's Court of Cassation ends the trial of members of the group behind the March 2014 attack in a village near Manama, for which seven others had received life sentences.
The Emirati officer was part of the Saudi-led Gulf force which rolled into Bahrain in March 2011 to boost Bahraini security forces in quelling a month-long protest dominated by island's Shiite majority.
Hundreds of Shiites have been arrested and put on trial since the crackdown on the protests that took their cue from Arab Spring uprisings and called for a constitutional monarchy with an elected prime minister in the Sunni-ruled Gulf state.