The version of the Rosh states: he should study by himself in order not to lose his learning. The Reshash cites a braisa in Meseches Semochos which states that the reason he should learn is in order not to forget his learning. The Rambam states this reason as well.
A question is asked: Doesn’t every Jew have an obligation to study Torah? What is different about a person that has been placed in cherem, that he should study Torah only because of the concern that otherwise he might forget his learning?
A similar question is asked (Igros Moshe Y”D 2:110; B’Toroso Yehegeh p. 185) on the Rambam in Hilchos Talmud Torah (1:10) who states: Until when is a person obligated to study Torah? Until the day he dies. The Rambam cites a verse in the Torah and concludes: And if there is a time that he will not be engrossed in studying Torah, he will forget his learning. Why is that necessary? Would a person be exempt from studying Torah if he is confident that his learning will not be forgotten?