Probably it is because of the existence of magic. Foot soldiers have the power of a field artillery but at a close range, so healers/field medics act like mechanics to keep them always in shape to fight.
She is also a kid and can only cast spells for a limited amount of time, so pretty much she's expendable in a way... it sucks, but it is what it is.
And the commander probably saw her and went:
"I need a replacement."
yoink
Still would make sense to groom her into a proper healer and the chief medic herself lamented this in an earlier chapter. But considering the medical staff seems to be completely addicted to potions, it isn't that much better.
Would be fun to see a different elite squad. Is the commander and his behaviour the norm or the exception?
Though, if this was done, it would probably be a "father to his men loses a whole unit to show that the commander's ruthlessness is acceptable" or something. War stories tend to be that way.