It seems to me a pashut Hilluq.
When one is reading the M'gillah on Purim no special Kavvanah is required. Why not? Because the default presumption is one is fulfilling the mitzvah via action, therefore a lack of Kavvanah is NOT m'akkeiv.
However, when one is WRITING the M'gillah, the kavvanah is apparently not for the sake of HEARING the M'gillah - ergo a positive "kivvein libbo" is a must.
Same s'vara with mit'aseiq with Shofar.
Any action which is manifestly and obviously fulfilling a Mitzvah -presupposes intention
Any action that appears to be tangential to the Mitzvah would require a positive intention to fulfill one's obligation.
Miitzot tz'richot kavvanah? Yes and no. It depends on the context.