Shabbat 3A

בבא דרישא, פטור ומותר לא קתני. אלא בבא דסיפא, דפטור אבל אסור ⁠— קשיא.

the first section of the Mishnah speaks of cases in which the one performing the actions is exempt from punishment by Torah law, and even by rabbinic law he is ab initio permitted to perform those actions. When the poor person or homeowner neither lifted nor placed the object, i.e., the object was placed into or removed from their hands by others, their role is insignificant. Therefore, it was not taught in the Mishnah, and those cases were not factored into the total number of acts of carrying from domain to domain. However, with regard to the latter section of the Mishnah, where the person performing those actions is exempt by Torah law, but his actions are prohibited by rabbinic law, it is difficult. Since the rabbis prohibited those actions, they should be included in the total in the Mishnah, which should be 12, not 8.

מי איכא בכולי שבת ״פטור ומותר״?! והאמר שמואל: כל פטורי דשבת פטור אבל אסור, בר מהני תלת דפטור ומותר: צידת צבי, וצידת נחש, ומפיס מורסא!

Incidentally, the Talmud wonders: Is there, in all the halakhot of Shabbat, an act for which the Mishnah deems one exempt and the act is permitted? Didn’t Shmuel say: With regard to all exempt rulings in the halakhot of Shabbat, although one who performs the action is exempt by Torah law, his action is prohibited by rabbinic law. This applies to all cases except for these 3 cases for which one is exempt and he is permitted to perform the action: Trapping a deer, where he does not actually trap it, rather he sits in the entrance of a house that a deer had previously entered on its own, preventing its exit; and trapping a poisonous snake because of the danger that it poses; and one who drains an abscess, meaning one who lances the boil of pus and drains the liquid from it. If so, the cases in the first section of our Mishnah, where the ruling is exempt, must be understood as exempt but prohibited.