Fridge Brilliance: Assuming that the practice of deferring convictions in exchange for military service is brought back into practice in the future (in Real Life, this has been specifically outlawed in the United States for several decades), it seems unlikely that someone would be sentenced to become a commissioned military officer and a Space Fighter pilot, as opposed to some kind of enlisted footsoldier or laborer. It's possible that the judge was manipulating the law to get Cooper Hawkes into a far better situation than the one he originated in (homeless and nearly murdered in an alleyway, only to be arrested by the cops afterwards).
Fridge Logic: Expensively trained pilots do not get used as footsoldiers. The makers were obviously aware of this as there's one scene where McQueen chews them out for complaining about taking on this role, saying it's their job to do whatever task they're given, and citing the old Marine Corps chestnut "Every marine is first and foremost a rifleman".
Fridge Horror: How bad must the casualties be if the Marines are using officers who are trained for flying space fighters for ground combat that any enlisted grunt can and should be doing in their place?
It's stated that for the first half of the series, the war is going very badly for Earth forces, hence the above. By the second half, things are going much better thanks in no small part to the 58th, who have become somewhat legendary, so it's possible they continued to be given ground combat missions both for their proven combat effectiveness and the high impact on overall morale.