Yesterday’s comments stole my thunder on this topic. Women should theoretically be exempt from reading megillah because it is a mitzvah which is zman gerama, but since af hein hayu b’oso hanes they indeed are obligated. What is the conceptual difference between Rashi’s understanding of af hein, that women shared the experience of the Purim miracle, and Rashbam’s understanding, that they initiated the miracle? My thought matched that of Anon1 - this issue perhaps depends on the Brisker Rav's (stencil Archin 3) chakira: does the sevara of af hein hayu b’oso hanes simply remove the potential exemption of zman gerama, or is it itself a new categorical obligation? According to Rashi, women and men are equally obligated in megillah reading based on their shared experience of the miracle of Purim. Af hein simply removes the exemption from women. According to Rashbam, af hein is a categorically new obligation which stems from women being the initiators of the miracle. The Brisker Rav suggests that this explains the dispute in Rishonim whether women can read megillah for men. If they share the same obligation, theoretically this would be acceptable; however, if the obligation of men and women are categorically different in nature, a woman cannot be motzi a man in keriah. (Side question: all the Rishonim assume that men can be motzi women. If the chiyuvim are categorically different, why should this be true? I am not sure of the answer…)
I recently heard a shiur on this issue from R’ Friedman, R.Y. of Mesivta Rambam, and his explanation matched that suggested in the comment by R’ Tal Benschar. According to Rashi, megillah reading is simple expression of gratitude for being saved. According to Rashbam, reading megillah is much more than that – it includes an acknowledgment of the specific vehicle through which Hashem’s hashgacha was demonstrated to the world. The reading serves almost as a re-enactment of the event of the miracle itself, and is therefore limited to the participants (or their representatives) in the events which unfolded.