They could always have allowed conscription, or at least civilian service in lieu, based on Matthew 5:41. Being compelled to carry a Roman soldier's pack for a mile was a form of conscription.
But the GB wanted martyrs. Just as in the 90s they gloated about the deaths of children who refused blood transfusions, they wanted loyalty to the point of being imprisoned by young, healthy adults. It was also apparently a great way to recruit Bethelites.
After people like Terry sacrificed their youth and gained criminal records for themselves, the WT never acknowledged them or rewarded them in any way for their faithfulness. Then they quietly changed the rules...