The machines are deliberately provided for entertainment. Anyone can become addicted to gambling, including soldiers. However, gamblers can become addicted for various reasons. If soldiers can control themselves in gambling, then wouldn't gambling just become entertainment for those who are far from family and public entertainment in the city?
Whether it is ethical or not depends on the perspective we view it from. I am also sure that soldiers may not have much time to keep playing the gambling machines provided. They have a time agenda that must be done routinely and with discipline.