The Stalinist penal system can be profitably studied with the same sociological tools we use to analyze penal structures elsewhere. It contained large numbers of common criminals serving relatively short sentences, many of whom were released each year and replaced by newly convicted persons. It included a wide variety of sanctions, including non-custodial ones. For most of those drawn into it, it was in fact a penal system: a particularly harsh, cruel, and arbitrary one, to be sure, but not necessarily a one-way ticket to oblivion for the majority of inmates.
You admittedly don't know the full story but are for some reason sure of their innocence? You know plenty of prisoners in the USSR were imprisoned for stuff you'd get imprisoned for in any country, right?