To atone for certain transgressions, the torah (Vayikra 4) mandates different offerings depending on the status of the person who transgressed. Horayot 3:3, talking about the king or the kohein gadol, says:
If they transgressed before they were appointed, and afterwards they were appointed, they are regarded as regular people [with respect to the transgression]. Rabbi Shimon said: if their sin came to their knowledge before they were appointed they are liable, but if after they were appointed they are exempt.
The first sentence, presumably the position of the rabbis, makes sense to me: when they transgressed they were laymen, so they bring the offering of a layman. (On the other end, though, once a kohein gadol, always a kohein gadol -- even if he steps down and then transgresses, he brings the larger offering.)
Rabbi Shimon's position, however, confuses me. He doesn't say that, in contrast to the first statement, they bring the offering that matches their current position; rather, he says they're exempt entirely. Why is that? It seems counter to pretty much everything the torah says about how we atone for transgressions.
I reviewed the g'mara on this passage but don't understand the argument. It starts by reasoning from the "after" case and talks about different kinds of errors for the ruler, and also talks about heavenly plagues (punishments), but I'm having trouble following it.
What is Rabbi Shimon's reasoning (or anyone else's who holds like him) for exempting someone entirely from atoning for a transgression?