They weren't considered criminals, they were sent to detention for clearing because otherwise it would've been easy for spies to get into army ranks. Of course,

They weren't considered criminals, they were sent to detention for clearing because otherwise it would've been easy for spies to get into army ranks. Of course, it sounds straight-forward in theory, but during war times the conditions in the detention were awful, and the whole Soviet system of gulags was very demanding of labor, so there weren't many incentives to get cleared.