Because 'rah rah' we all love the military. It doesn't cost them anything and they get the PR of everyone boarding a plane hearing about how much they love the military.

It might be a deploy. Military sometimes use civilian transport to cut costs.

Because it's the least that can be done for people willing to take substandard pay for risking their lives in literal battle zones. You can only really use it when you're traveling on orders, meaning being transferred to somewhere away from your family or when you're in full uniform. Most service members I know only use it when being deployed.