Many became informants because they were approached by agents asking them to tell them everything they knew about a target. Pretending to not know anything was risky. If they discovered that you didn't tell them about something you should have known, it was your ass that got sent to a re-education camp. If they as much as suspected that you were withholding information, you may have found your job suddenly relocated to some forgotten corner of the country.

If you think, i'm exaggerating. My grandfather, a prominent writer, was exiled to teach elementary school in far-off village for a few years after one of the ruling elites decided that his latest play was unsympathetic to Communist ideals.