And why shouldn't our service members enjoy some of the things they'd be able to at home when possible?
But yeah, it's very bad optics to essentially take their paycheck back. But one of the reasons they are playing the slots are to win money. So if you make it cash neutral, it's really pointless.
And you could instead just set up a free-play arcade. Or even a quarter to help with maintenance costs. But people look at video games and gambling differently. One is for children and one is for adults.
But then again, these people signed up to be told what to do, what to eat, etc. I would say it's fair that the military could say that gambling while enlisted is not allowed and shut down the slot rooms.