A career in the military is not the same as a career anywhere else. In their situation the ability to be bribed by the enemy is too great a risk. In industry that's what NDAs and non-compete clauses are for.
Who the hell wants a staff full of debt-burdened carrot-chasers who couldn't get the job they really wanted?
Employees with a mountain of debt are less likely to cause trouble for their employer, as the employee's debt must be serviced, especially education debt in the US.
It is, for all intents and purposes, a rather relaxed version of indentured servitude, considering the overall lack of mobility in the job market.
Their idea of an offer you can't refuse is an offer... and you'd better not refuse.