Obstruction of justice can be less bad for you than the offense concealed, so if the offense is bad enough, risking punishment for that may not be irrational if it offers a real chance of concealing another offense.
I might be giving CA more credit than they're due in the smarts department, but considering that I'm just a dude making up scenarios, I find this particular one pretty plausible. And assuming the worst seems like a sensible approach in this case.