That's like saying child labor was never mandatory. If labor is your only possible source of income, [0] and there are heavy incentives to perform labor, then people will perform labor. It being voluntary does not change the fact that it is heavily coercive (especially when your work pool has the lowest legal rights within the citizenry—a similar situation with child rights in the past).

[0]: And if you don't have money, you cannot access the commissary nor even make a simple phone call.