I was taught that the Halachah is that the Torah designates that only Males are supposed to do the “Shaklah V’Taryah” of learning Gemorah. (for example, Shulchan Aruch 241:6, Taz (#4), Maharil (199), Rambam Talmud Torah 1:13, Sotah 20a)
It is permitted to teach Females, the extracted CONCLUSIONS of those discussions, the Halachah L’Maaseh, so that Females will know the proper Halachah to fulfill the Torah.
The Mitzvah of learning Torah is on Males; therefore, the concept of Bitul Torah applies to them.