R. Shimon says: From its place (i.e., from the place where Scripture makes him liable to kareth), it is to be inferred (that once he is smitten he is exempt from kareth), it being written (Vayikra 18:29) "For all who commit any of these abominations (in illicit relations) — the souls that do it are to be cut off (i.e., kareth) from their people," and (Ibid. 18:5) "And you shall keep My statutes and My judgments, which, if a man do, he shall live by them," (— followed by (6) "A man, a man, to all the kin of his flesh you shall not come near to uncover nakedness.")
From this ("he shall live by them") we infer that if one sits (passively) and does not commit transgression, (in this instance, the kareth transgression of illicit relations), he receives the reward of one who (actively) performs a mitzvah ([How much more so is the smitten one exempt from kareth!)].