Why do active military get to board with children and disabled people?

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.

You can just walk in after business class and they’ll let you in without hassling about you getting ahead of your next-to-toilet class.

Guaranteed overhead.

Lol

This isn’t always true, fortunately. I’ve seen JetBlue attendants yell at people who kept trying to jump in boarding classes. Line cutters are scum.

The real scum are the people that sit in the back and fill in the overhead bins up front.