Torah Temimah → בְּכוֹרוֹת ד׳ א׳
English: Whoever is subject to human firstborn is subject to impure-animal firstborn; kohanim and Leviim, not being subject to human firstborn, are not subject to peter chamor.
Kohanim/Leviim exempt from peter chamor.
The hekesh links human and impure-animal firstborn; those free of one are free of the other.