Why would becoming king or kohein gadol exempt one from atoning for past sins?

Question

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?

Answer 1

The Yerushalmi to this Mishnah brings two explanations for R’ Shimon’s opinion:

חברייא אמרין טעמא דר"ש משם שהגדולה מכפרת אמר ר' יוסי שאין חטאו וידיעתו שוין

The Chaveirim say: The reason for R’ Shimon is because his stature atones. R’ Yosi said: because his sin and knowledge aren’t the same.

That is to say: According to the Chaveirim, that he becomes a Nasi in and of itself atones for his sin; a Karban Chatas is unnecessary (compare Yerushalmi Bikkurim 3:3). According to R’ Yosi, it’s because the sin and knowledge thereof didn’t take place in the same category (i.e. the former was before he became Nasi, while the latter was after he became Nasi); since both are needed to qualify for any particular type of Karban Chatas, he doesn’t fit in any of the boxes which would obligate him in any particular type of Karban Chatas (this seems to be how Rashi and Tiferes Yisrael understand this opinion).

The Yerushalmi notes that a practical difference between these two approaches would be whether R’ Shimon argues on the previous Mishnah as well, where he sinned as a Nasi but only realized it after he was demoted. According to the Chaveirim, he would still be obligated, as there was nothing with which to gain atonement, while according to R’ Yosi, he would still be exempt, as the two still didn’t happen in the same stature.