Unless it's been implemented by the commander/drillmaster. And possibly even though, it's not really a policy.

Look at what you wrote below:

"I keep running into lower level/midlevel ragers who say they can't rank until they kill more mages"

"Are they saying they aren't allowed to? Is there some sort of policy in place preventing them from ranking until they get these kills?"

1) Did they say 'they can't rank until they kill more mages' or 'they don't want to'?
2) Why don't you find out in character?
3) There's nothing that 'prevents them from ranking'. They can rank if they want to. They choose not to. There's nothing that's hardcoded.

If you look into the situation (which you failed to do), a better question would be 'why don't they rank above their current level?'

To which the answer(s) would be really really obvious to you.

BTW - your question is really freaking whiney and screams 'LOOK AT ME! I'M LAZY AND I WANT TO CLUTTER THE BOARD WITH NONSENSICAL CRAP!'

Even the way you phrase the question seems to be a flame against someone(s). If you can't see it, I'm not here to point it out.

Think about it.