The issue is that ozone purification cleans the water at the source, but it's free to pick up contamination in the pipes as it makes its way to the tap; chlorin

The issue is that ozone purification cleans the water at the source, but it's free to pick up contamination in the pipes as it makes its way to the tap; chlorination works right up until the water is drunk. The downside is that chlorinated water does not taste great. If one has a good pipe system, the ozonated water is fine. If there's any worry, though, chlorinated is safer. I'm not surprised that a military base would choose the safer alternative: no-one ever got fired for ensuring that the water is safe to drink.