What is the reason for the different rulings in Islamic law regarding clitoral stimulation (permissible if the wife asks her husband during intercourse, impermissible if the husband asks his wife to rub her clitoris herself, and impermissible in masturbation), even though the method and purpose are the same?

There is a significant difference between a woman enjoying herself [masturbation], which is forbidden, as it falls under the Almighty's saying: ﴿But whoever seeks beyond that, then those are the transgressors﴾, and her enjoying her husband, which is permissible, due to the Almighty's saying: ﴿except with their spouses.﴾ Thus, she may enjoy him in every way except for intercourse during menstruation and anal sex.